diff --git a/.gitignore b/.gitignore
index d515d75..2060ead 100644
--- a/.gitignore
+++ b/.gitignore
@@ -23,6 +23,8 @@ simulator/sample_media/**/*.m4a
 # Local-only candidate review galleries (re-buildable; not shipped content).
 simulator/static/review.html
 simulator/static/review_audio.html
+simulator/static/review_binaural.html
+simulator/static/review_music.html
 
 # Editor annotation/comment-thread metadata (not project content)
 .threads/
diff --git a/docs/audio-candidate-pool.md b/docs/audio-candidate-pool.md
index 140c3b4..6073fa1 100644
--- a/docs/audio-candidate-pool.md
+++ b/docs/audio-candidate-pool.md
@@ -50,7 +50,13 @@ recordings (NASA/NOAA) keep the "machines reshaping perception" thesis literal.
 | 🏖 coast | `coast/waves` | Sea waves + tern calls | BigSoundBank #0267 — https://bigsoundbank.com/sea-waves-and-seagulls-s0267.html (`/UPLOAD/mp3/0267.mp3`) | 57s | CC0 |
 | 🕳 abyss | `abyss/whale` | NE Pacific blue-whale "AB call" | NOAA PMEL — https://www.pmel.noaa.gov/acoustics/multimedia/NETS_bluWhale.wav | 13s | PD (NOAA) |
 
-## Music (the Audio-dropdown "Music" option, shipped 2026-07-01)
+## Music (the Audio-dropdown "Music" option)
+
+> **SUPERSEDED 2026-07-02:** the single ethereal loop below was replaced by a
+> **rotating crossfading playlist of mellow drumless synth** (Kevin MacLeod, CC BY) —
+> see `docs/music-candidate-pool.md`. The ethereal-loop record is kept for history.
+
+### (history) ethereal loop — shipped 2026-07-01, then replaced
 
 **One** ethereal ambient loop, played the same at every altitude (altitude-independent —
 NOT the per-altitude idea the "cosmos/music" / "Frozen Star" rows above reserved; those
diff --git a/docs/binaural-candidate-pool.md b/docs/binaural-candidate-pool.md
new file mode 100644
index 0000000..6e7e1bd
--- /dev/null
+++ b/docs/binaural-candidate-pool.md
@@ -0,0 +1,110 @@
+# Binaural / entrainment candidate pool — stress-relief "Music" tracks (2026-07-02)
+
+> **PARKED (2026-07-02):** the operator dropped the binaural direction in favor of
+> **mellow chordal synth music** — see `docs/music-candidate-pool.md`. This file is kept
+> as the record of the binaural exploration (and its NC-licensing dead-ends).
+
+
+Candidates for evolving the **Music** audio option into a stress-relief set of
+**binaural-beat / brainwave-entrainment** tracks that **crossfade between each
+other** (not a single loop). Operator brief: "binaural beats for stress relief."
+
+**All 11 below are CC0** (public domain — no attribution legally required; we credit
+anyway as courtesy), **all stereo**, all sourced from freesound.org and license-
+verified by fetching each page. Review gallery (gitignored, local-only, like
+`review_audio.html`): `simulator/static/review_binaural.html` → open
+`/review_binaural.html` with the sim server running. It streams each track's public
+HQ preview so you can audition + tick keepers.
+
+## Caveats (operator, read before picking)
+
+- **True binaural (#1–7) needs headphones.** The beat *is* the L/R frequency
+  difference, so on speakers it collapses to a steady carrier tone (no entrainment).
+  **#8–11** are ambient sub-bass / pulsing drones that keep the calm feel on speakers.
+- **Band coverage:** heavy on **delta (0.5–1.5 Hz)** + one clean **theta (4 Hz, #2)**.
+  **No CC0 alpha (8–12 Hz)** track surfaced — widen to CC-BY (FMA/ccMixter) if wanted.
+- **No clean CC0/CC-BY isochronic or monaural** (the speaker-correct entrainment type)
+  was found — every hit was NC/SA or unlicensed (see rejects). For speaker playback,
+  lean on #8–11 for the "beat" feel.
+- **Previews vs originals:** the `cdn.freesound.org/…-hq.mp3` URLs are public streamable
+  previews (verified live, HTTP 206). The **originals** (WAV/AIF/full-MP3) need a free
+  freesound login; CC0 permits self-hosting, so for keepers download the originals and
+  host on R2 — do not hot-link the CDN in production.
+
+## Candidates
+
+| # | id | title | artist | source page | license | dur | type | beat / carrier |
+|---|---|---|---|---|---|---|---|---|
+| 1 | `fs75309`  | 60-64-delta | iantm | https://freesound.org/people/iantm/sounds/75309/ | CC0 | 0:30 | true-binaural | 4 Hz · 60/64 Hz |
+| 2 | `fs457654` | Binaural Beats (Theta Waves 4 Hz) | Fabrizio84 | https://freesound.org/people/Fabrizio84/sounds/457654/ | CC0 | 30:00 | true-binaural | 4 Hz theta |
+| 3 | `fs803742` | 1.23456789 Hz binaural beat (69L/70.23R) | itchytooth | https://freesound.org/people/itchytooth/sounds/803742/ | CC0 | 60:00 | true-binaural | 1.23 Hz delta · 69/70.23 Hz |
+| 4 | `fs414271` | Binaural Delta 440/440.5 Hz | pbabin | https://freesound.org/people/pbabin/sounds/414271/ | CC0 | 10:00 | true-binaural | 0.5 Hz · 440/440.5 Hz |
+| 5 | `fs414272` | Binaural Delta 500/501.5 Hz | pbabin | https://freesound.org/people/pbabin/sounds/414272/ | CC0 | 10:00 | true-binaural | 1.5 Hz · 500/501.5 Hz |
+| 6 | `fs414273` | Binaural Delta 52/52.5 Hz | pbabin | https://freesound.org/people/pbabin/sounds/414273/ | CC0 | 10:00 | true-binaural | 0.5 Hz · 52/52.5 Hz |
+| 7 | `fs414274` | Binaural Delta 60/61.5 Hz | pbabin | https://freesound.org/people/pbabin/sounds/414274/ | CC0 | 10:00 | true-binaural | 1.5 Hz · 60/61.5 Hz |
+| 8 | `fs856225` | Delta Pulse Subharmonic Drone (MANTICE) | bassimat | https://freesound.org/people/bassimat/sounds/856225/ | CC0 | 5:00 | ambient+beats (speaker-ok) | delta sub-bass (procedural) |
+| 9 | `fs854874` | Void Monolith Subharmonic Drone (MANTICE) | bassimat | https://freesound.org/people/bassimat/sounds/854874/ | CC0 | 5:00 | ambient (speaker-ok) | dark sub-drone |
+| 10 | `fs860122` | Expanding & Vibrating Stereo Pulsating Drone (MANTICE) | bassimat | https://freesound.org/people/bassimat/sounds/860122/ | CC0 | 2:00 | ambient+beats (speaker-ok) | slow stereo pulsation |
+| 11 | `fs493524` | Pulse Breath *(lower pref)* | phylobates | https://freesound.org/people/phylobates/sounds/493524/ | CC0 | 1:40 | ambient binaural | octaves of G / G♭ |
+
+Preview MP3s (for the review page) follow `https://cdn.freesound.org/previews/<dir>/<id>_<uploader>-hq.mp3`
+— exact URLs are embedded in `review_binaural.html`.
+
+## Round 2 — musical / evolving (the "not boring" set, 2026-07-02)
+
+Operator found the pure-tone set (#1–7) too plain. **Reality:** truly *musical* tracks
+with a **confirmed** binaural layer are almost all NC (see rejects). Best legitimate
+options — pick for how they *sound*; treat binaural as a bonus:
+
+| id | title | artist | source | license | dur | binaural? |
+|---|---|---|---|---|---|---|
+| `fs856223` | Theta Gateway Sacred Drone (MANTICE) | bassimat | https://freesound.org/people/bassimat/sounds/856223/ | CC0 | 5:00 | capable (theta band; not certified on render) |
+| `fs856221` | Raga of Stillness Sacred Drone (MANTICE) | bassimat | https://freesound.org/people/bassimat/sounds/856221/ | CC0 | 5:00 | capable (Δ/θ/α/β optional) |
+| `fs856220` | Ice Cathedral Sacred Drone (MANTICE) | bassimat | https://freesound.org/people/bassimat/sounds/856220/ | CC0 | 5:00 | capable |
+| `fs856224` | Vowel of the Deep Sacred Drone (MANTICE) | bassimat | https://freesound.org/people/bassimat/sounds/856224/ | CC0 | 5:00 | capable |
+| `fs856222` | Stellar Vowel Sacred Drone (MANTICE) | bassimat | https://freesound.org/people/bassimat/sounds/856222/ | CC0 | 5:00 | capable |
+| `fs856329` | Harmonic Chamber High-Frequency Drone (MANTICE) | bassimat | https://freesound.org/people/bassimat/sounds/856329/ | CC0 | 5:00 | capable |
+| `fs854438` | Immersive Ethereal Generative Ambient (Floating Harmonics) | bassimat | https://freesound.org/people/bassimat/sounds/854438/ | CC0 | 11:33 | no (richest single piece) |
+| `fma_cosmic` | Cosmic Waves | HoliznaCC0 | https://freemusicarchive.org/music/holiznacc0/space-sleep-meditation/cosmic-waves/ | CC0 | 33:04 | no (long-form melodic pads) |
+| `fma_dream` | DreamScape | HoliznaCC0 | https://freemusicarchive.org/music/holiznacc0/space-sleep-meditation/dreamscape/ | CC0 | 21:59 | no |
+| `fma_rain` | Rain / Sleep / Meditation | HoliznaCC0 | https://freemusicarchive.org/music/holiznacc0/space-sleep-meditation/rain-sleep-meditation/ | CC0 | 14:35 | no (rain + pads) |
+| `ccm_shagrugge` | How to Get Rid of a Dream (real imagination mix) | shagrugge | http://dig.ccmixter.org/files/shagrugge/12629 | CC BY 3.0 | ~ | yes-ish (binaural-filtered vocals; experimental) |
+
+- The MANTICE **Sacred Drone** series is the composed extension of Round-1 #8–10 (same
+  generator; the generator *does* binaural entrainment across Δ/θ/α/β, but the individual
+  freesound renders don't certify the beat is on — bassimat could confirm/render on request).
+- **HoliznaCC0** (FMA, CC0) = the strongest pure-*music* options; no binaural layer.
+- FMA `files.freemusicarchive.org/...` URLs verified live (206) — safe to self-host.
+  The **ccMixter** shagrugge URL is **hotlink-protected** (403) — download in a browser and self-host.
+
+### Opt-in (outside the CC0/CC-BY rule — needs operator OK)
+
+- **"Hang Binaural" — Mindseye** — https://freemusicarchive.org/music/MindsEye/single/hang-binaural
+  — **CC BY-SA 4.0**. The single best musical+*confirmed*-binaural fit (hang-drum music +
+  real binaural beats, 3:36, stereo), but ShareAlike copyleft. Add only if SA is acceptable.
+
+## Rejected / could not verify
+
+- **archive.org "Binaural Beats – Alpha/Beta/Theta/Delta"** & **"Isochronic Tones &
+  Binaural Beats Combined"** (loopool / Jean-Paul Garnier) — **CC BY-NC-SA 3.0** (NC).
+  The most promising isochronic set — rejected for the NC clause.
+- **archive.org "Om Alpha Brainwave Entrainment Loop"** (Entraino / KS Chauhan;
+  freesound chauhans/100370) — **CC BY-NC 3.0**. Rejected (NC).
+- **archive.org "1hr 30 Min Theta Binaural Beat (7 Hz)"** (Chari G) — no license shown.
+  Rejected (unverifiable).
+- **freesound "deadthetawaves.wav"** (NoiseCollector/43295) — license fine (**CC BY 3.0**)
+  but "evil sounding loop of the living dead" — rejected for *fit*, not license. Back
+  pocket only if an unsettling deep-scale track is ever wanted.
+- **TunePocket / dialedggsound / miraclefrequencies / audio.com "Theta 7Hz"** —
+  "royalty-free"/commercial pages, murky redistribution, no per-track CC license.
+- **Round 2 NC drops** (great content, non-commercial licence): "Fear Nothing (Relaxation
+  Mix)" — Jeris (CC BY-NC), "On the Earth" — Quarkstar (CC BY-NC), "Timeless (Binaural
+  Beatmix)" — duckett (CC BY-NC), "Ambient Meditation" — Scott Holmes (CC BY-NC). ccMixter's
+  binaural tag is overwhelmingly NC; only shagrugge (CC BY) survived.
+
+## Next
+
+Operator auditions `review_binaural.html`, ticks keepers (Copy picks). Then we wire the
+playback: the **Music** source becomes a multi-track set that crossfades between the kept
+tracks (architecture decision — replace vs. join the ethereal loop — deferred to that
+step; see [[audio-music-option]]).
diff --git a/docs/music-candidate-pool.md b/docs/music-candidate-pool.md
new file mode 100644
index 0000000..fc42edc
--- /dev/null
+++ b/docs/music-candidate-pool.md
@@ -0,0 +1,129 @@
+# Music candidate pool — mellow chordal synth (2026-07-02)
+
+The chosen direction for the **Music** audio option: **mellow, soothing synth music with
+real chord movement** ("many chords" — progressions that evolve, not static drones or
+binaural tones). Supersedes the binaural exploration ([[binaural-candidate-pool]] — parked).
+We'll use **several** tracks and **crossfade between them** (not loop one).
+
+All **CC0 or CC-BY** (verified by fetching each page + a live HTTP 206 check on every audio
+URL). Local review gallery (gitignored, like the other review pages):
+`simulator/static/review_music.html` → open `/review_music.html` with the sim server running.
+
+**Honesty flag:** licenses + URLs are verified; the "harmony" notes are inferred from
+genre/reputation, not a by-ear check — audition before final selection.
+
+## FINAL selection — WIRED 2026-07-02
+
+After a Round-4 drumless pass, the operator's **final Music set = 5 Kevin MacLeod tracks**
+(CC BY 4.0), now **shipped as a rotating crossfading playlist**:
+
+**Music for Manatees · Dewdrop Fantasy · Eastminster · Overheat · Concentration**
+
+Wiring: `app.js` `MUSIC_PLAYLIST` — the Music source plays one track and every
+`MUSIC_ROTATE_MS` (5 min) crossfades to a random OTHER track (`pickNextIndex`, never an
+immediate repeat), altitude-independent, on the two `#aud`/`#aud-b` elements. Files
+loudness-normalized + transcoded under the 25MB LFS limit by `build_audio_media.py`
+(`MUSIC`), committed via LFS; the single ethereal loop was **retired**. Credited on
+`credits.html` (CC BY attribution). Earlier candidates below kept for record.
+
+*(History: the operator first kept Dream Culture + Silver Blue Light, then — after the
+drumless Round-4 pass — chose the 5 long-form/floaty tracks above instead.)*
+
+## Two license families
+
+- **HoliznaCC0 = CC0** — zero attribution burden (simplest for the credits page).
+- **Kevin MacLeod = CC BY 4.0** — must credit, e.g. *"Chill Wave by Kevin MacLeod —
+  incompetech.com — Licensed under Creative Commons: By Attribution 4.0."* The credits page
+  already exists, so this is fine.
+- **Self-host everything** on R2 for the install — the `files.freemusicarchive.org` and
+  `incompetech.com` URLs are live/hot-linkable now but shouldn't be depended on in prod.
+
+## Round 4 — drumless, chord-moving (the refined set, 2026-07-02)
+
+All **Kevin MacLeod, CC BY 4.0**, **drumless confirmed from his per-track `pieces.json`
+instrument metadata** (tonal instruments only — no drum kit/percussion), verified live (206).
+This is the authoritative candidate list; the lo-fi below (beat-driven) is dropped.
+
+*(mp3 host `https://incompetech.com/music/royalty-free/mp3-royaltyfree/<Title>.mp3`; source page
+`https://incompetech.com/music/royalty-free/index.html?isrc=<code>`.)*
+
+**Best matches — metered, chords clearly move** (safest bets, like the two keepers):
+| title | isrc | dur | instruments | harmony |
+|---|---|---|---|---|
+| Impact Lento | USUAN1100619 | 3:31 | Synths, Choir | "several striking chord progressions" |
+| Dreams Become Real | USUAN1500027 | 6:50 | Piano, Synths | dreamy, layered, evolving (closest to Dream Culture) |
+| Aretes | USUAN1100325 | 4:36 | Piano, Synths, Choir | warm major, clear motion |
+| When The Wind Blows | USUAN1100717 | 4:26 | Guitar, Synths | lilting, calm (Silver-Blue-Light recipe) |
+| Reawakening | USUAN1400017 | 3:34 | Piano, Cello | warm resolving progression |
+| Almost New | USUAN1800008 | 3:23 | Piano, Basses | gentle moving harmony (documented) |
+
+**Long-form / floaty — drumless but slower-moving** (audition for "boring" risk):
+| title | isrc | dur | instruments | note |
+|---|---|---|---|---|
+| Music for Manatees | USUAN1400009 | 17:36 | Piano, EP, Strings, Synth | slow consonant shifts |
+| Clean Soul | USUAN1300033 | 5:07 | Electric Piano, Bass | spacious, sparser |
+| Soaring | USUAN1600040 | 6:20 | Synths | evolving pads, mystical-calm |
+| Dewdrop Fantasy | USUAN1700001 | 34:58 | Synths, Rain | slowly changing, lower density |
+| Eastminster | USUAN1100719 | 6:45 | Plucked Strings, Synths | drone + koto, some tension — least chordal |
+| Overheat | USUAN1100630 | 7:03 | Flute, Synth, Violin | trance-y minimalism |
+| Concentration | — | 29:53 | Synths, EP | floaty, lower density |
+
+Reference: the kept **Dream Culture** is tagged "Piano, **Percussion**" (subtle beat); **Silver Blue
+Light** is "Guitar, Synths" (drumless). The Round-4 picks are strictly tonal-only. A ~40-track
+filtered pool of more drumless-calm-chordal KM tracks exists if a bigger rotation is wanted.
+
+**Rejected this round (percussion / not soothing):** Angel Share (Drums), Dreamer (Percussion,
+bpm 100), Lightless Dawn (Percussion + unnerving), Anamalie (Percussion + dark), Chill Wave &
+all HoliznaCC0 lo-fi (beat-driven).
+
+## Candidates — earlier mellow-synth round (lo-fi DROPPED, kept for record)
+
+Ranked by chord movement.
+
+### Lo-fi / dreamy — HoliznaCC0 (CC0)
+
+| id | title | source page | mp3 (verified 206) | dur | harmony |
+|---|---|---|---|---|---|
+| `tokyo`   | Tokyo Sunset | https://freemusicarchive.org/music/holiznacc0/public-domain-lofi/tokyo-sunset-lofi-peaceful-soft/ | .../Xnd9Hr5AVzB68IlWcImKtXPlwCePD2G2m8ZFSVj4.mp3 | 2:18 | jazzy extended-chord progression |
+| `reverie` | Dreamy Reverie | https://freemusicarchive.org/music/holiznacc0/public-domain-lofi/dreamy-reverie-lofi-nostalgic-chill/ | .../Bo2XWazV4kHpnPksjJGRRLr16EmOE44NZnMITZaB.mp3 | 2:23 | moving 7th/9th chords |
+| `complic` | Complicated Feelings | https://freemusicarchive.org/music/holiznacc0/public-domain-lofi/complicated-feelings-lofi-dreamy-relaxed/ | .../jEFkzWNxxtz76vjPoUOjZALHvdK3HkVMQAnGpgTn.mp3 | 2:39 | bittersweet changes |
+| `drift`   | Peaceful Drift | https://freemusicarchive.org/music/holiznacc0/public-domain-lofi/peaceful-drift-lofi-nostalgic-calm/ | .../SQvtLguk6S1VSthv0oXWycoB6ipUS0pt8jzAxxPq.mp3 | 2:08 | calm nostalgic progression |
+| `currents`| Calm Currents | https://freemusicarchive.org/music/holiznacc0/public-domain-lofi/calm-currents-lofi-relax-calm/ | .../4rKapZUMNnNSPAOvpjlfSH6B5Ib8rgEWdvjnM7C6.mp3 | 2:27 | flowing chords |
+| `moon`    | Moon Unit | https://freemusicarchive.org/music/holiznacc0/public-domain-lofi/moon-unit-lofi-reflection-dreamy/ | .../CbNZO1QUuJq1f50RHzZ5kykNj1hdqT04UaWOYSNf.mp3 | 2:52 | reflective dreamy chords |
+| `mist`    | Into The Mist | https://freemusicarchive.org/music/holiznacc0/public-domain-lofi/into-the-mist-lofi-calm-relaxed/ | .../cI5iv9Xga0OOvGsS7mGZn9BlTshZdzAqLW7bmArA.mp3 | 2:09 | calm progression |
+| `wave`    | Wave Maker | https://freemusicarchive.org/music/holiznacc0/public-domain-lofi/wave-maker-lofi-dreamy-retro/ | .../SbPJTWMnBmT5yBsXP4jLfp6RsvWLhfOnbw2PxXol.mp3 | 3:05 | warm analog retro chords |
+
+*(mp3 host: `https://files.freemusicarchive.org/storage-freemusicarchive-org/tracks/`)*
+
+### Calming synth — Kevin MacLeod (CC BY 4.0 — credit required)
+
+| id | title | source page | mp3 (verified 206) | dur | harmony |
+|---|---|---|---|---|---|
+| `chillwave`  | Chill Wave | https://archive.org/details/ChillWave_781 | https://incompetech.com/music/royalty-free/mp3-royaltyfree/Chill%20Wave.mp3 | 3:57 | clear repeating 4-chord cycle |
+| `enchanted`  | Enchanted Journey | https://freemusicarchive.org/music/Kevin_MacLeod/Calming | https://incompetech.com/music/royalty-free/mp3-royaltyfree/Enchanted%20Journey.mp3 | 4:29 | many changes, warm major |
+| `dreamcult`  | Dream Culture | https://freemusicarchive.org/music/Kevin_MacLeod/Calming | https://incompetech.com/music/royalty-free/mp3-royaltyfree/Dream%20Culture.mp3 | 3:34 | evolving chord sequence |
+| `silverblue` | Silver Blue Light | https://freemusicarchive.org/music/Kevin_MacLeod/Calming | https://incompetech.com/music/royalty-free/mp3-royaltyfree/Silver%20Blue%20Light.mp3 | 5:50 | slow, deliberate changes |
+
+### Spares
+
+- **Clean Soul** — Kevin MacLeod — CC BY — 5:06 — `https://incompetech.com/music/royalty-free/mp3-royaltyfree/Clean%20Soul.mp3`
+- **Old Age** — HoliznaCC0 — CC0 — 3:33 — https://freemusicarchive.org/music/holiznacc0/only-in-the-milky-way-part-1/old-age/
+- **Deja Vu** — HoliznaCC0 — CC0 — 2:04 — https://freemusicarchive.org/music/holiznacc0/only-in-the-milky-way-part-1/deja-vu/
+- More Kevin MacLeod (all CC BY 4.0, incompetech): Dreamer, Reawakening, Lightless Dawn,
+  Almost New, Angel Share. The whole **HoliznaCC0 "Public Domain Lofi"** album is 42 CC0
+  tracks in this vibe — a ready-made rotation if wanted.
+
+## Rejected / not used
+
+- **Kevin MacLeod baroque/classical "public-domain" pieces** — the *composition* is PD but
+  incompetech states the *recording* is CC BY 4.0; harpsichord/orchestral, off-brief anyway.
+- **NC finds** (great but non-commercial): Scott Holmes "Ambient Meditation" (CC BY-NC),
+  ccMixter chordal ambient (Jeris/Quarkstar/duckett — CC BY-NC).
+- **"Hang Binaural" — Mindseye** — CC BY-SA (ShareAlike), excluded per the CC0/CC-BY rule.
+
+## Next
+
+Operator auditions `review_music.html`, ticks keepers (Copy picks). Then wire the **Music**
+source to a multi-track set that crossfades between the kept tracks (replaces the single
+ethereal loop; see [[audio-music-option]]). Kept CC-BY tracks get a credit line on
+`credits.html`; all kept files self-hosted on R2.
diff --git a/simulator/build_audio_media.py b/simulator/build_audio_media.py
index fafc287..0d8ba75 100644
--- a/simulator/build_audio_media.py
+++ b/simulator/build_audio_media.py
@@ -1,42 +1,62 @@
-"""Production pass for the 5 per-altitude soundtracks + the single Music loop
-(audio spec §5.3; Music layer per docs/audio-candidate-pool.md). The audio
-analogue of build_pool_manifest.py --media: read the sourced ambience clips
-(re-downloadable per docs/audio-candidate-pool.md), and make each a seamless
-loudness-normalized loop.
+"""Production pass for the 5 per-altitude soundtracks + the Music playlist
+(audio spec §5.3; Music per docs/music-candidate-pool.md). The audio analogue of
+build_pool_manifest.py --media: read the sourced clips (re-downloadable per the
+candidate-pool docs) and make each web-ready.
 
-Media is gitignored; this script is the reproducible record. Outputs land under
-simulator/sample_media/audio/<scale>/<name>.loop.mp3, served at /media/audio/...
-Run:  python simulator/build_audio_media.py
+- Soundtracks: seamless loudness-normalized loops -> audio/<scale>/<name>.loop.mp3.
+- Music: the mellow drumless synth playlist (Kevin MacLeod, CC BY 4.0) — NOT looped
+  (the client crossfades between tracks). Each is loudness-normalized and transcoded
+  to a bitrate that keeps it under the ~25MB git-LFS/nginx limit (ambient tolerates
+  it) -> audio/music/<name>.mp3. Put the downloaded original at audio/music/<name>.src.mp3
+  (URLs below) first; this transcodes it.
 
-(White-noise is deferred — the live Audio control is a simple on/off soundtrack
-toggle. `tools.pipeline.audio_run.generate_white_noise` remains for when it's
-wanted, but no noise bed is built here.)
+Media is committed via git-LFS; this script is the reproducible record.
+Run:  python -m simulator.build_audio_media
 """
 
 from __future__ import annotations
 
+import subprocess
 from pathlib import Path
 
 from tools.pipeline.audio_run import process_soundtrack
+from tools.pipeline.run import resolve_ffmpeg
 
 AUDIO = Path(__file__).parent / "sample_media" / "audio"
 
 # scale -> (sourced raw file, production output file), both under audio/<scale>/.
 # Sources per docs/audio-candidate-pool.md (the "✓ KEEP" picks).
-#
-# The `music` entry is the Audio-dropdown "Music" layer (docs/audio-candidate-pool.md
-# § Music): ONE ethereal ambient loop played the same at every altitude — not a
-# per-altitude soundtrack. It lands at audio/music/ethereal.loop.mp3, served at
-# /media/audio/music/ethereal.loop.mp3 (client fallback: app.js MUSIC_FALLBACK).
 SOURCES: dict[str, tuple[str, str]] = {
     "cosmos": ("pillars.mp3", "pillars.loop.mp3"),
     "orbit": ("spaceamb.mp3", "spaceamb.loop.mp3"),
     "coast": ("waves.mp3", "waves.loop.mp3"),
     "reef": ("soundscape.mp3", "soundscape.loop.mp3"),
     "abyss": ("whale.wav", "whale.loop.mp3"),
-    "music": ("ethereal.mp3", "ethereal.loop.mp3"),
 }
 
+# Music playlist (docs/music-candidate-pool.md § Round 4): name -> (incompetech source
+# URL, target bitrate). Kevin MacLeod, CC BY 4.0 — credited on credits.html. Drumless,
+# chord-moving ambient; the operator's picks. Longer tracks get a lower bitrate to stay
+# under the LFS size limit. Client playlist: app.js MUSIC_PLAYLIST.
+_KM = "https://incompetech.com/music/royalty-free/mp3-royaltyfree/"
+MUSIC: dict[str, tuple[str, str]] = {
+    "manatees": (_KM + "Music%20for%20Manatees.mp3", "96k"),
+    "dewdrop": (_KM + "Dewdrop%20Fantasy.mp3", "80k"),
+    "eastminster": (_KM + "Eastminster.mp3", "128k"),
+    "overheat": (_KM + "Overheat.mp3", "128k"),
+    "concentration": (_KM + "Concentration.mp3", "80k"),
+}
+
+
+def _transcode(src: Path, dst: Path, bitrate: str, ff: str) -> Path:
+    """Loudness-normalize + transcode to mp3 at `bitrate` (no looping)."""
+    subprocess.run(
+        [ff, "-y", "-hide_banner", "-loglevel", "error", "-i", str(src),
+         "-af", "loudnorm", "-c:a", "libmp3lame", "-b:a", bitrate, "-ar", "44100", str(dst)],
+        check=True,
+    )
+    return dst
+
 
 def main() -> None:
     for scale, (raw, out) in SOURCES.items():
@@ -47,6 +67,15 @@ def main() -> None:
         dst = process_soundtrack(src, AUDIO / scale / out)
         print(f"  produced {dst.relative_to(AUDIO.parent)}")
 
+    ff = resolve_ffmpeg()
+    for name, (url, bitrate) in MUSIC.items():
+        src = AUDIO / "music" / f"{name}.src.mp3"
+        if not src.exists():
+            print(f"  SKIP music/{name}: source missing ({src}) — download {url}")
+            continue
+        dst = _transcode(src, AUDIO / "music" / f"{name}.mp3", bitrate, ff)
+        print(f"  produced {dst.relative_to(AUDIO.parent)} @ {bitrate}")
+
 
 if __name__ == "__main__":
     main()
diff --git a/simulator/e2e/tests/audio-source.spec.ts b/simulator/e2e/tests/audio-source.spec.ts
index 9df942b..631f188 100644
--- a/simulator/e2e/tests/audio-source.spec.ts
+++ b/simulator/e2e/tests/audio-source.spec.ts
@@ -2,7 +2,7 @@ import { test, expect, Page } from "@playwright/test";
 
 // The Audio-source feature (docs/superpowers/specs/2026-07-01-audio-music-option-design.md):
 // the old 0–10 "Audio" slider becomes "Volume", and a new Audio <select> chooses the
-// source — None / Soundtrack / Music. Music is ONE ethereal loop played the same at
+// source — None / Soundtrack / Music. Music is a rotating playlist played the same at
 // every altitude (no per-altitude crossfade).
 
 async function enter(page: Page) {
@@ -47,27 +47,30 @@ test.describe("Audio source dropdown + Volume rename", () => {
     await expect(page.locator("#audio")).toBeEnabled();
   });
 
-  test("Music plays one ethereal loop that persists across an altitude change", async ({ page }) => {
+  const MUSIC_RE = /audio\/music\/(manatees|dewdrop|eastminster|overheat|concentration)\.mp3/;
+
+  test("Music plays a playlist track that persists across an altitude change", async ({ page }) => {
     await enter(page);
     await selectSource(page, "music");
     let urls = await audUrls(page);
-    expect(urls.a).toContain("music/ethereal.loop.mp3");
-    // Move the dial: Music is altitude-independent, so #aud stays on the music loop
-    // (it does NOT switch to a per-altitude soundtrack).
+    expect(urls.a).toMatch(MUSIC_RE);
+    // Move the dial: Music is altitude-independent, so #aud stays on a music track
+    // (it does NOT switch to a per-altitude soundtrack). Default rotate interval is
+    // 5 min, so it's the SAME track here.
     await wheelOnStage(page, 120);
     await wheelOnStage(page, 120);
     await page.waitForTimeout(300);
     urls = await audUrls(page);
-    expect(urls.a).toContain("music/ethereal.loop.mp3");
+    expect(urls.a).toMatch(MUSIC_RE);
   });
 
-  test("Soundtrack loads a per-altitude ambience, not the music loop", async ({ page }) => {
+  test("Soundtrack loads a per-altitude ambience, not a music track", async ({ page }) => {
     await enter(page);
     await selectSource(page, "soundtrack");
     await page.waitForTimeout(200);
     const urls = await audUrls(page);
     const loaded = urls.a + "|" + urls.b;
     expect(loaded).toContain("/audio/");
-    expect(loaded).not.toContain("music/ethereal.loop.mp3");
+    expect(loaded).not.toContain("/audio/music/");
   });
 });
diff --git a/simulator/e2e/tests/auto-advance.spec.ts b/simulator/e2e/tests/auto-advance.spec.ts
new file mode 100644
index 0000000..fb5e4cf
--- /dev/null
+++ b/simulator/e2e/tests/auto-advance.spec.ts
@@ -0,0 +1,55 @@
+import { test, expect, Page } from "@playwright/test";
+
+// Auto-advance: after the dial sits idle for the interval, the altitude drifts one
+// step deeper (descend), wrapping past the deepest. A default-on toggle disables it.
+// The e2e injects a short interval via window.__hefAutoAdvanceMs before load.
+
+async function boot(page: Page, intervalMs: number) {
+  await page.addInitScript((ms) => { (window as any).__hefAutoAdvanceMs = ms; }, intervalMs);
+  await page.goto("/");
+  await page.waitForFunction(() => (window as any).__hefReady === true, null, { timeout: 45_000 });
+  await page.locator("#welcome-launch").click();
+  await page.waitForSelector("#welcome", { state: "detached", timeout: 10_000 });
+}
+
+const idx = (page: Page) => page.evaluate(() => (window as any).__hefState().ringIndex as number);
+
+test("auto-advances one altitude (descending) after the idle interval", async ({ page }) => {
+  await boot(page, 1200);
+  const n = await page.evaluate(() => (window as any).__hefState && (document.querySelectorAll(".dial-label").length));
+  const start = await idx(page);
+  // Wait past the interval + the morph animation for the landing.
+  await page.waitForFunction((s) => (window as any).__hefState().ringIndex !== s, start, { timeout: 8000 });
+  const after = await idx(page);
+  expect(after).toBe((start + 1) % n);   // descended exactly one, wraps
+});
+
+test("Auto-advance off keeps the altitude parked", async ({ page }) => {
+  // Disable BEFORE launch so the idle timer never arms (deterministic — no race with
+  // the first interval firing during boot).
+  await page.addInitScript(() => { (window as any).__hefAutoAdvanceMs = 800; });
+  await page.goto("/");
+  await page.waitForFunction(() => (window as any).__hefReady === true, null, { timeout: 45_000 });
+  await page.evaluate(() => { (document.querySelector("#auto-advance") as HTMLInputElement).checked = false; });
+  await page.locator("#welcome-launch").click();
+  await page.waitForSelector("#welcome", { state: "detached", timeout: 10_000 });
+
+  const start = await idx(page);
+  await page.waitForTimeout(2200);       // well past two 800ms intervals
+  expect(await idx(page)).toBe(start);   // never moved
+});
+
+test("the idle countdown resets on interaction (no advance right after a manual turn)", async ({ page }) => {
+  await boot(page, 1500);
+  // Immediately before the first interval elapses, interact (keyboard step). That step
+  // itself advances by 1; the reset then means no AUTO advance fires for another
+  // full interval — so shortly after, we should still be at exactly that +1.
+  await page.waitForTimeout(900);
+  const before = await idx(page);
+  await page.locator("#dial").focus();
+  await page.locator("#dial").press("ArrowDown");            // manual descend + reset
+  await page.waitForFunction((b) => (window as any).__hefState().ringIndex !== b, before, { timeout: 4000 });
+  const manual = await idx(page);
+  await page.waitForTimeout(900);                            // < the 1500ms interval
+  expect(await idx(page)).toBe(manual);                      // countdown reset → no extra auto step yet
+});
diff --git a/simulator/e2e/tests/mood-grade.spec.ts b/simulator/e2e/tests/mood-grade.spec.ts
new file mode 100644
index 0000000..1f97027
--- /dev/null
+++ b/simulator/e2e/tests/mood-grade.spec.ts
@@ -0,0 +1,46 @@
+import { test, expect, Page } from "@playwright/test";
+
+// Bug: the Dark/Light Mood grade was dropped DURING an altitude morph — the morph
+// showed ungraded ("normal") and the grade snapped back only on landing. Both render
+// paths were at fault: the WebGL canvas set filter "none" while `busy`, and the
+// fallback set #vid's filter "none" when a morph loaded. The grade must ride through.
+
+async function boot(page: Page) {
+  await page.goto("/");
+  await page.waitForFunction(() => (window as any).__hefReady === true, null, { timeout: 45_000 });
+  await page.locator("#welcome-launch").click();
+  await page.waitForSelector("#welcome", { state: "detached", timeout: 10_000 });
+  await expect(page.locator("#scale-name")).toBeVisible();
+}
+
+test("Dark Mood grade stays applied to the morph while scrubbing", async ({ page }) => {
+  await boot(page);
+  // Drive Mood hard dark; wait for the debounced render to build the grade.
+  await page.evaluate(() => {
+    const m = document.querySelector("#mood") as HTMLInputElement;
+    m.value = "-4";
+    m.dispatchEvent(new Event("input", { bubbles: true }));
+  });
+  await page.waitForTimeout(300);
+
+  // Begin a drag → a morph plays (busy = true). Sample the VISIBLE morph surface:
+  // the paint canvas (WebGL path) or #vid (fallback). #vid-loop is hidden mid-morph,
+  // so we deliberately exclude it.
+  const box = (await page.locator("#dial").boundingBox())!;
+  const cx = box.x + box.width / 2, cy = box.y + box.height / 2;
+  await page.mouse.move(cx, cy - 30);
+  await page.mouse.down();
+  await page.mouse.move(cx + 18, cy - 24, { steps: 8 });
+  await page.waitForFunction(() => (document.querySelector("#vid") as HTMLVideoElement).currentTime > 0, null, { timeout: 5000 });
+
+  const filters = await page.evaluate(() => [
+    (document.querySelector("#paint") as HTMLElement).style.filter,
+    (document.querySelector("#vid") as HTMLElement).style.filter,
+  ]);
+  await page.mouse.up();
+
+  // Dark grade is brightness(<1)… — at least one visible surface must carry it.
+  // Before the fix both were "none" here.
+  const graded = filters.some((f) => /brightness\(/.test(f) && !/brightness\(1(\.0+)?\)/.test(f));
+  expect(graded, `visible morph surface must carry the dark grade; saw ${JSON.stringify(filters)}`).toBe(true);
+});
diff --git a/simulator/e2e/tests/music-playlist.spec.ts b/simulator/e2e/tests/music-playlist.spec.ts
new file mode 100644
index 0000000..8dc5d95
--- /dev/null
+++ b/simulator/e2e/tests/music-playlist.spec.ts
@@ -0,0 +1,57 @@
+import { test, expect, Page } from "@playwright/test";
+
+// Music is a rotating playlist that crossfades to a random OTHER track on a timer
+// (docs/music-candidate-pool.md). The e2e injects short rotate/xfade intervals via
+// window.__hefMusicRotateMs / __hefMusicXfadeMs before load.
+
+async function boot(page: Page, rotateMs: number, xfadeMs: number) {
+  await page.addInitScript(([r, x]) => {
+    (window as any).__hefMusicRotateMs = r;
+    (window as any).__hefMusicXfadeMs = x;
+  }, [rotateMs, xfadeMs]);
+  await page.goto("/");
+  await page.waitForFunction(() => (window as any).__hefReady === true, null, { timeout: 45_000 });
+  await page.locator("#welcome-launch").click();
+  await page.waitForSelector("#welcome", { state: "detached", timeout: 10_000 });
+}
+
+// The track on the louder (active) music element.
+async function activeTrack(page: Page): Promise<string> {
+  return await page.evaluate(() => {
+    const a = document.querySelector("#aud") as HTMLAudioElement;
+    const b = document.querySelector("#aud-b") as HTMLAudioElement;
+    const el = b.volume > a.volume ? b : a;
+    return el.dataset.url || "";
+  });
+}
+
+test("Music crossfades to a DIFFERENT playlist track after the rotate interval", async ({ page }) => {
+  await boot(page, 1200, 300);
+  await page.selectOption("#audio-source", "music");
+  await page.waitForTimeout(200);
+  const first = await activeTrack(page);
+  expect(first).toMatch(/audio\/music\/\w+\.mp3/);
+  // Wait past one rotation (1200ms) + the crossfade (300ms) + buffer.
+  await page.waitForFunction((f) => {
+    const a = document.querySelector("#aud") as HTMLAudioElement;
+    const b = document.querySelector("#aud-b") as HTMLAudioElement;
+    const el = b.volume > a.volume ? b : a;
+    return (el.dataset.url || "") !== f && (el.dataset.url || "").includes("/audio/music/");
+  }, first, { timeout: 6000 });
+  const second = await activeTrack(page);
+  expect(second).toMatch(/audio\/music\/\w+\.mp3/);
+  expect(second).not.toBe(first);          // rotated to another track
+});
+
+test("Music keeps rotating (a second rotation lands on a track again)", async ({ page }) => {
+  await boot(page, 900, 250);
+  await page.selectOption("#audio-source", "music");
+  const seen = new Set<string>();
+  for (let i = 0; i < 3; i++) {
+    await page.waitForTimeout(1300);
+    seen.add(await activeTrack(page));
+  }
+  // Over 3 rotations we should have heard more than one distinct track.
+  expect([...seen].every((u) => /audio\/music\/\w+\.mp3/.test(u))).toBe(true);
+  expect(seen.size).toBeGreaterThan(1);
+});
diff --git a/simulator/e2e/tests/panel-layout.spec.ts b/simulator/e2e/tests/panel-layout.spec.ts
new file mode 100644
index 0000000..8d6674a
--- /dev/null
+++ b/simulator/e2e/tests/panel-layout.spec.ts
@@ -0,0 +1,25 @@
+import { test, expect, Page } from "@playwright/test";
+
+// Guard: the "Video speed" control label must stay on ONE line in the Output panel
+// (it was wrapping to two rows at the old panel width).
+
+async function enter(page: Page) {
+  await page.goto("/");
+  await page.waitForFunction(() => (window as any).__hefReady === true, null, { timeout: 45_000 });
+  await page.locator("#welcome-launch").click();
+  await page.waitForSelector("#welcome", { state: "detached", timeout: 10_000 });
+}
+
+test('the "Video speed" label does not wrap to a second line', async ({ page }) => {
+  await enter(page);
+  const info = await page.evaluate(() => {
+    const span = document.querySelector('.panel .play-speed span[data-i18n="speed.label"]') as HTMLElement;
+    const cs = getComputedStyle(span);
+    let lh = parseFloat(cs.lineHeight);
+    if (Number.isNaN(lh)) lh = parseFloat(cs.fontSize) * 1.3; // "normal"
+    return { text: span.textContent, height: span.getBoundingClientRect().height, lineHeight: lh };
+  });
+  expect(info.text).toBe("Video speed");
+  // A wrapped label is ~2× a single line; allow slack but stay well under two lines.
+  expect(info.height).toBeLessThan(info.lineHeight * 1.6);
+});
diff --git a/simulator/e2e/tests/url-params.spec.ts b/simulator/e2e/tests/url-params.spec.ts
new file mode 100644
index 0000000..d42fd36
--- /dev/null
+++ b/simulator/e2e/tests/url-params.spec.ts
@@ -0,0 +1,47 @@
+import { test, expect, Page } from "@playwright/test";
+
+// Shareable deep-link: ?audio=none|soundtrack|music presets the Audio source on load
+// (and ?vol=0..10 the master volume), so a link opens straight into a chosen sound mode.
+
+async function ready(page: Page) {
+  await page.waitForFunction(() => (window as any).__hefReady === true, null, { timeout: 45_000 });
+}
+async function launch(page: Page) {
+  await page.locator("#welcome-launch").click();
+  await page.waitForSelector("#welcome", { state: "detached", timeout: 10_000 });
+}
+const srcVal = (page: Page) => page.locator("#audio-source").inputValue();
+
+test("?audio=music preselects the Music source and survives launch", async ({ page }) => {
+  await page.goto("/?audio=music");
+  await ready(page);
+  expect(await srcVal(page)).toBe("music");   // set at init, before launch
+  await launch(page);
+  expect(await srcVal(page)).toBe("music");
+});
+
+test("?audio=soundtrack preselects Soundtrack", async ({ page }) => {
+  await page.goto("/?audio=soundtrack");
+  await ready(page);
+  expect(await srcVal(page)).toBe("soundtrack");
+});
+
+test("?audio=none preselects None and disables Volume", async ({ page }) => {
+  await page.goto("/?audio=none");
+  await ready(page);
+  expect(await srcVal(page)).toBe("none");
+  await expect(page.locator("#audio")).toBeDisabled();
+});
+
+test("an invalid ?audio value is ignored (stays default Soundtrack)", async ({ page }) => {
+  await page.goto("/?audio=banana");
+  await ready(page);
+  expect(await srcVal(page)).toBe("soundtrack");
+});
+
+test("?vol sets the master volume for the shared link", async ({ page }) => {
+  await page.goto("/?audio=music&vol=7");
+  await ready(page);
+  await launch(page);
+  expect(await page.locator("#audio").inputValue()).toBe("7");
+});
diff --git a/simulator/e2e/tests/welcome-audio.spec.ts b/simulator/e2e/tests/welcome-audio.spec.ts
new file mode 100644
index 0000000..4a157cf
--- /dev/null
+++ b/simulator/e2e/tests/welcome-audio.spec.ts
@@ -0,0 +1,43 @@
+import { test, expect, Page } from "@playwright/test";
+
+// The welcome (opening) screen mirrors the panel controls. It must offer the Audio
+// SOURCE choice (None/Soundtrack/Music) too — so a visitor can pick their sound mode
+// before launching — carried into the session on entry.
+
+async function ready(page: Page) {
+  await page.waitForFunction(() => (window as any).__hefReady === true, null, { timeout: 45_000 });
+}
+
+test("the welcome screen has an Audio source dropdown with 3 options", async ({ page }) => {
+  await page.goto("/");
+  await ready(page);
+  await expect(page.locator("#welcome")).toBeVisible();
+  const opts = page.locator("#welcome-audio-source option");
+  await expect(opts).toHaveCount(3);
+  await expect(opts).toHaveText(["None", "Soundtrack", "Music"]);
+});
+
+test("choosing Music on the welcome screen carries into the session", async ({ page }) => {
+  await page.goto("/");
+  await ready(page);
+  await page.selectOption("#welcome-audio-source", "music");
+  await page.locator("#welcome-launch").click();
+  await page.waitForSelector("#welcome", { state: "detached", timeout: 10_000 });
+  expect(await page.locator("#audio-source").inputValue()).toBe("music");
+  await page.waitForTimeout(300);
+  const url = await page.evaluate(() => (document.querySelector("#aud") as HTMLAudioElement).dataset.url || "");
+  expect(url).toMatch(/audio\/music\/\w+\.mp3/);
+});
+
+test("welcome None greys out the welcome Volume slider", async ({ page }) => {
+  await page.goto("/");
+  await ready(page);
+  await page.selectOption("#welcome-audio-source", "none");
+  await expect(page.locator("#welcome-audio")).toBeDisabled();
+});
+
+test("?audio=music preselects Music on the welcome screen too", async ({ page }) => {
+  await page.goto("/?audio=music");
+  await ready(page);
+  expect(await page.locator("#welcome-audio-source").inputValue()).toBe("music");
+});
diff --git a/simulator/sample_media/audio/music/concentration.mp3 b/simulator/sample_media/audio/music/concentration.mp3
new file mode 100644
index 0000000..1cebc12
--- /dev/null
+++ b/simulator/sample_media/audio/music/concentration.mp3
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:8c5fb794efbf0b8e6c59c6723e1da29b61ca2b5b48c882419909797d7ddb650d
+size 17929636
diff --git a/simulator/sample_media/audio/music/dewdrop.mp3 b/simulator/sample_media/audio/music/dewdrop.mp3
new file mode 100644
index 0000000..80530de
--- /dev/null
+++ b/simulator/sample_media/audio/music/dewdrop.mp3
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:b96e9051603066ae8b43213a231d309b9a0234f275d4a68fb0d9a54a92d48acb
+size 20979975
diff --git a/simulator/sample_media/audio/music/eastminster.mp3 b/simulator/sample_media/audio/music/eastminster.mp3
new file mode 100644
index 0000000..29d7afd
--- /dev/null
+++ b/simulator/sample_media/audio/music/eastminster.mp3
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:86047b903f50275d6578b8e4051bbb75d2784cb39f3719f7e7b2b81eb58c83a2
+size 6482165
diff --git a/simulator/sample_media/audio/music/ethereal.loop.mp3 b/simulator/sample_media/audio/music/ethereal.loop.mp3
deleted file mode 100644
index 0c9fbc4..0000000
--- a/simulator/sample_media/audio/music/ethereal.loop.mp3
+++ /dev/null
@@ -1,3 +0,0 @@
-version https://git-lfs.github.com/spec/v1
-oid sha256:dd4df4eac246061979d4959a95dbefa7405a57d45f6d8d1e06d1d98dad644a26
-size 3742100
diff --git a/simulator/sample_media/audio/music/ethereal.mp3 b/simulator/sample_media/audio/music/ethereal.mp3
deleted file mode 100644
index d76ac08..0000000
--- a/simulator/sample_media/audio/music/ethereal.mp3
+++ /dev/null
@@ -1,3 +0,0 @@
-version https://git-lfs.github.com/spec/v1
-oid sha256:3829adf9ca64784268b21a061f0b1cf49b2b14139949f8a2ea3f33ffd04c3949
-size 4923905
diff --git a/simulator/sample_media/audio/music/manatees.mp3 b/simulator/sample_media/audio/music/manatees.mp3
new file mode 100644
index 0000000..c7b6abe
--- /dev/null
+++ b/simulator/sample_media/audio/music/manatees.mp3
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:80547ef4a54a9879e1cf94f827b19da350feda1e55dd86876485dff620b5dfd1
+size 12673735
diff --git a/simulator/sample_media/audio/music/overheat.mp3 b/simulator/sample_media/audio/music/overheat.mp3
new file mode 100644
index 0000000..9115e48
--- /dev/null
+++ b/simulator/sample_media/audio/music/overheat.mp3
@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:e0fa4796031f146b7f8bef55d613212c3a0f6b3de05bd48233e4229ed0a244e0
+size 7066594
diff --git a/simulator/static/app.js b/simulator/static/app.js
index 18004dc..71729e2 100644
--- a/simulator/static/app.js
+++ b/simulator/static/app.js
@@ -451,11 +451,13 @@ function paintLoop() {
     if (paint.width !== w || paint.height !== h) { paint.width = w; paint.height = h; }
     gl.viewport(0, 0, w, h);
     try { gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGB, gl.RGB, gl.UNSIGNED_BYTE, src); } catch (_) {}
-    // No painterly restyle during a ring transition (busy) — show it raw.
+    // No painterly DREAM during a ring transition (busy) — show the morph crisp.
+    // The Dark/Light mood GRADE, however, must ride through the transition too
+    // (else the morph shows ungraded and the grade snaps back only on landing).
     gl.uniform1f(uAmt, busy ? 0 : dreamIntensity);
     gl.uniform2f(uTexel, 1 / w, 1 / h);
     gl.drawArrays(gl.TRIANGLES, 0, 3);
-    paint.style.filter = busy ? "none" : gradeFilter;
+    paint.style.filter = gradeFilter;
   }
   // Animate per-frame: re-place keyframed tracks AND re-evaluate time-windowed
   // labels (a label enters/leaves frame as the loop plays) against playback time.
@@ -846,6 +848,7 @@ function autoScrub(targetPos, perAltMs = PER_ALTITUDE_MS) {
 let wheelAccum = 0, wheelTimer = null;
 function onWheel(e) {
   e.preventDefault();
+  bumpAutoAdvance();
   wheelAccum += e.deltaY;
   clearTimeout(wheelTimer);
   wheelTimer = setTimeout(() => {
@@ -855,6 +858,27 @@ function onWheel(e) {
   }, 90);
 }
 
+// --- Auto-advance: after the dial sits idle for AUTO_ADVANCE_MS, drift one altitude
+// deeper (descend; past the deepest it wraps back to the top), a slow kiosk tour. Any
+// manual interaction restarts the idle countdown; a default-on toggle disables it.
+// (E2E injects a short interval via window.__hefAutoAdvanceMs before load.)
+const AUTO_ADVANCE_MS = (typeof window !== "undefined" && window.__hefAutoAdvanceMs) || 60000;
+let autoAdvanceTimer = null;
+function autoAdvanceOn() { const el = $("auto-advance"); return el ? el.checked : true; }
+function scheduleAutoAdvance() {
+  clearTimeout(autoAdvanceTimer);
+  autoAdvanceTimer = null;
+  if (!autoAdvanceOn() || !videoEverOn) return;   // only while the experience is running
+  autoAdvanceTimer = setTimeout(onAutoAdvance, AUTO_ADVANCE_MS);
+}
+function onAutoAdvance() {
+  // Skip a beat (but keep the cadence) if the visitor is mid-drag or a morph is playing.
+  if (autoAdvanceOn() && videoEverOn && !dialDrag && !busy) autoScrub(Math.round(pos) + 1);
+  scheduleAutoAdvance();
+}
+// Restart the idle countdown on any manual interaction (drag/wheel/key/knob).
+function bumpAutoAdvance() { if (videoEverOn) scheduleAutoAdvance(); }
+
 // --- Altitude knob: the endless rotary encoder, drawn as a turnable dial ---
 // cosmos (highest) sits at the top; turning CLOCKWISE descends through the scales
 // (cosmos → orbit → coast → reef → abyss) and past the deepest WRAPS back to the
@@ -983,7 +1007,10 @@ function rebuildSegment(lo, enteredFrom) {
     overlay.style.opacity = "0";
     affectLayer.style.opacity = "0";
     tint.style.opacity = "0";
-    vid.style.filter = "none";
+    // WebGL path: the graded canvas sits on top, so #vid stays raw (no double-grade).
+    // Fallback path: #vid IS the visible surface, so it must carry the mood grade
+    // through the morph (setting "none" here was the "morph ignores Mood" bug).
+    vid.style.filter = paintOK ? "none" : gradeFilter;
     vid.loop = false;
     if (vid.dataset.morph !== file) {
       vid.dataset.morph = file;
@@ -1059,6 +1086,7 @@ function setPos(next) {
 function onDialDown(e) {
   if (!ring || ring.scales.length < 2) return;
   e.preventDefault();
+  bumpAutoAdvance();
   dialDrag = { lastAng: dialAngle(e), accum: 0, moved: 0, target: e.target, restPos: pos };
 }
 function onDialMove(e) {
@@ -1085,6 +1113,7 @@ function onDialUp(e) {
   // short way to the CLOSEST one and settle there (frac 0 locks). The result is the
   // same as a tap/label-click: a grab-and-release always lands on a discrete altitude.
   autoScrub(Math.round(pos));
+  bumpAutoAdvance();
 }
 
 // Click a label → auto-scrub the SHORTEST signed way around the ring to that scale.
@@ -1115,7 +1144,7 @@ function onDialKey(e) {
     case "End":  jumpToScale(ring.scales.length - 1); break;
     default: handled = false;
   }
-  if (handled) e.preventDefault();
+  if (handled) { e.preventDefault(); bumpAutoAdvance(); }
 }
 
 // --- Dev Mode: pool picker + live analysis, all under one toggle ---
@@ -1381,6 +1410,28 @@ function syncVolumeDisabled() {
   if (slider) slider.disabled = audioSource() === "none";
 }
 
+// Shareable deep-link: ?audio=none|soundtrack|music presets the Audio source on load
+// (and ?vol=0..10 the master volume), so a link opens straight into a chosen sound mode
+// — e.g. share …/?audio=music with someone who'd prefer music over the soundtrack. The
+// source is set on the (welcome-hidden) panel dropdown; the launch gesture then applies
+// it. Invalid values are ignored (the default Soundtrack stands).
+function applyUrlAudioParams() {
+  const p = new URLSearchParams(location.search);
+  const src = (p.get("audio") || "").toLowerCase();
+  if (src === "none" || src === "soundtrack" || src === "music") {
+    const el = $("audio-source"); if (el) el.value = src;
+    const w = $("welcome-audio-source"); if (w) w.value = src;   // reflect on the opening screen too
+  }
+  const vol = p.get("vol");
+  if (vol !== null && vol !== "" && Number.isFinite(+vol)) {
+    const v = String(Math.max(0, Math.min(10, Math.round(+vol))));
+    // Seed BOTH the welcome mirror (carried into the panel on launch) and the panel
+    // slider, so the shared level survives entry.
+    const w = $("welcome-audio"); if (w) w.value = v;
+    const a = $("audio"); if (a) a.value = v;
+  }
+}
+
 // The element currently carrying the most signal — for the diagnostic readout,
 // which used to assume a single `aud`. With two crossfading elements either may be
 // the audible one, so report on whichever is louder (ties → aud).
@@ -1470,15 +1521,28 @@ function scaleAudioUrl(index) {
 // The current resting altitude's soundtrack url (kept for the status readout).
 function soundtrackUrl() { return scaleAudioUrl(ringIndex); }
 
-// The single continuous "Music" loop — one ethereal ambient track, played the
-// same at every altitude. Fallback constant mirrors SCALE_AUDIO_FALLBACK so it
-// works even against a server predating the manifest's music entry; routed through
-// mediaBase() for R2 (static) / local /media (dev), like scaleAudioUrl.
-const MUSIC_FALLBACK = "music/ethereal.loop.mp3";
-function musicUrl() { return mediaBase() + "audio/" + MUSIC_FALLBACK; }
+// --- Music: a rotating playlist of mellow synth tracks that CROSSFADE between each
+// other (not one looping track), altitude-independent — the same relaxing rotation at
+// every altitude. Every MUSIC_ROTATE_MS it crossfades to a random OTHER track (never an
+// immediate repeat), reusing the two audio elements. Intervals are injectable for tests.
+const MUSIC_PLAYLIST = [
+  "music/manatees.mp3", "music/dewdrop.mp3", "music/eastminster.mp3",
+  "music/overheat.mp3", "music/concentration.mp3",
+];
+const MUSIC_ROTATE_MS = (typeof window !== "undefined" && window.__hefMusicRotateMs) || 5 * 60 * 1000;
+const MUSIC_XFADE_MS  = (typeof window !== "undefined" && window.__hefMusicXfadeMs)  || 6000;
+function musicTrackUrl(i) {
+  return mediaBase() + "audio/" + MUSIC_PLAYLIST[HEFScrub.wrapIndex(i, MUSIC_PLAYLIST.length)];
+}
+let musicIdx = -1;     // index of the track on the active music element (-1 = not started)
+let musicEl = null;    // the element currently carrying the active music track
+let musicTimer = null; // rotation timer
 
 // The url the active source would play right now (for the diagnostic readout).
-function currentAudioUrl() { return audioSource() === "music" ? musicUrl() : soundtrackUrl(); }
+function currentAudioUrl() {
+  if (audioSource() === "music") return musicTrackUrl(musicIdx < 0 ? 0 : musicIdx);
+  return soundtrackUrl();
+}
 
 // Start (once) and set the volume of one element to a url. Safari needs the first
 // play() inside a user gesture; later programmatic plays reuse the unlocked element.
@@ -1511,19 +1575,47 @@ function assignElements(urlLo, urlHi) {
   return { elLo: aud, elHi: audB };
 }
 
-// Both elements fade to silence (None, or Volume 0).
+// Both elements fade to silence (None, or Volume 0). Also ends any music rotation.
 function silenceAll() {
+  stopMusic();
   fadeVolume(aud, 0, FADE_MS, () => aud.pause());
   fadeVolume(audB, 0, FADE_MS, () => audB.pause());
 }
 
-// Play the single continuous Music loop on `aud` at `vol`; fade the other out. No
-// altitude crossfade — the same track plays the same at every altitude.
-function playMusic(vol) {
-  ensurePlaying(aud, musicUrl(), vol);
-  fadeVolume(audB, 0, FADE_MS);
+function scheduleMusicRotate() {
+  clearTimeout(musicTimer);
+  musicTimer = setTimeout(rotateMusic, MUSIC_ROTATE_MS);
+}
+// Crossfade the active music element out and a random OTHER track in on the other element.
+function rotateMusic() {
+  if (audioSource() !== "music" || audioVol() === 0 || musicIdx < 0) { scheduleMusicRotate(); return; }
+  const next = HEFScrub.pickNextIndex(musicIdx, MUSIC_PLAYLIST.length, Math.random());
+  const nextEl = musicEl === aud ? audB : aud;
+  ensurePlaying(nextEl, musicTrackUrl(next), 0);                  // incoming track, silent...
+  fadeVolume(nextEl, audioVol(), MUSIC_XFADE_MS);                 // ...fade it in,
+  fadeVolume(musicEl, 0, MUSIC_XFADE_MS, () => musicEl.pause());  // ...fade the old out.
+  musicEl = nextEl; musicIdx = next;
+  scheduleMusicRotate();
   updateAudioStatus();
 }
+// Start the rotation (first call) or just re-apply the master volume to the active track.
+// The FIRST call runs inside the audio gesture, so we prime BOTH elements here to unlock
+// them on Safari (the timer-driven crossfade later plays the 2nd element outside a gesture).
+function startMusic(vol) {
+  if (musicIdx < 0) {
+    musicIdx = Math.floor(Math.random() * MUSIC_PLAYLIST.length);
+    musicEl = aud;
+    ensurePlaying(aud, musicTrackUrl(musicIdx), vol);
+    ensurePlaying(audB, musicTrackUrl(musicIdx), 0);   // prime + unlock the 2nd element (silent)
+    scheduleMusicRotate();
+  } else {
+    cancelFade(musicEl);
+    musicEl.volume = HEFScrub.clamp01(vol);
+  }
+  updateAudioStatus();
+}
+// Stop the rotation and reset (on leaving Music, or going silent).
+function stopMusic() { clearTimeout(musicTimer); musicTimer = null; musicIdx = -1; musicEl = null; }
 
 // Mid-segment scrub. Soundtrack: crossfade scale `loIndex` (gain 1-frac) against
 // loIndex+1 (gain frac), scaled by the master volume. Music: hold the single loop.
@@ -1531,7 +1623,8 @@ function playMusic(vol) {
 function blendAudio(loIndex, frac) {
   const plan = HEFScrub.audioPlan(audioSource(), audioVol(), frac);
   if (plan.mode === "silent") return;
-  if (plan.mode === "music") { playMusic(plan.vol); return; }
+  if (plan.mode === "music") { startMusic(plan.vol); return; }
+  stopMusic();                        // leaving Music for the soundtrack crossfade
   const urlLo = scaleAudioUrl(loIndex), urlHi = scaleAudioUrl(loIndex + 1);
   const { elLo, elHi } = assignElements(urlLo, urlHi);
   ensurePlaying(elLo, urlLo, plan.lo);
@@ -1545,7 +1638,8 @@ function blendAudio(loIndex, frac) {
 function restAudio(index) {
   const plan = HEFScrub.audioPlan(audioSource(), audioVol(), 0);
   if (plan.mode === "silent") { silenceAll(); return; }
-  if (plan.mode === "music") { playMusic(plan.vol); return; }
+  if (plan.mode === "music") { startMusic(plan.vol); return; }
+  stopMusic();                        // soundtrack → ensure the music rotation is stopped
   const url = scaleAudioUrl(index);
   const active = (audB.dataset.url === url) ? audB : aud;
   const idle = active === audB ? aud : audB;
@@ -1577,6 +1671,7 @@ function beginExperience() {
   updateAudioLevelLabel();
   applyAudio();
   applyPlaySpeed();
+  scheduleAutoAdvance();   // start the idle-drift countdown now that we're running
 }
 
 // Welcome screen ----------------------------------------------------------------
@@ -1587,11 +1682,18 @@ function applyWelcomeToPanel() {
   $("visual").checked = true;   // the welcome screen assumes Video on (no toggle)
   const wa = $("welcome-audio");
   if (wa) { $("audio").value = wa.value; updateAudioLevelLabel(); }
+  const ws = $("welcome-audio-source");   // carry the chosen source into the panel dropdown
+  if (ws) { $("audio-source").value = ws.value; syncVolumeDisabled(); }
 }
 function updateWelcomeAudioLabel() {
   const el = $("welcome-audio-val");
   if (el) el.textContent = String(+($("welcome-audio") || {}).value || 0);
 }
+// Volume is meaningless on None — grey the welcome slider too, mirroring the panel.
+function syncWelcomeVolumeDisabled() {
+  const src = $("welcome-audio-source"), slider = $("welcome-audio");
+  if (src && slider) slider.disabled = src.value === "none";
+}
 
 // Remove the welcome overlay (fade out) — the panel reveals via `body:has(#welcome)`.
 function enterSimulation() {
@@ -1624,6 +1726,9 @@ function initWelcome() {
   updateWelcomeAudioLabel();
   const wa = $("welcome-audio");
   if (wa) wa.addEventListener("input", updateWelcomeAudioLabel);
+  const ws = $("welcome-audio-source");
+  if (ws) ws.addEventListener("change", syncWelcomeVolumeDisabled);
+  syncWelcomeVolumeDisabled();
   const launch = $("welcome-launch");
   if (launch) launch.addEventListener("click", onLaunch);
 }
@@ -1641,7 +1746,9 @@ async function main() {
   initWelcome();              // wire the welcome-screen controls + Launch button
   renderScaleReadout();
   // Sliders stream on "input"; the toggles fire on "change".
-  for (const id of ["left", "right", "mood"]) $(id).addEventListener("input", debounced);
+  for (const id of ["left", "right", "mood"]) $(id).addEventListener("input", () => { bumpAutoAdvance(); debounced(); });
+  // Auto-advance toggle: (re)start or cancel the idle-drift countdown on change.
+  $("auto-advance").addEventListener("change", scheduleAutoAdvance);
   // Audio level dial: set the master soundtrack gain. Reconcile SYNCHRONOUSLY in this
   // input gesture so the first raise from 0 unlocks + plays on Safari (which blocks
   // play() outside a user gesture).
@@ -1650,7 +1757,10 @@ async function main() {
   // Audio source dropdown: reconcile SYNCHRONOUSLY in this change gesture so
   // switching to Soundtrack/Music unlocks + plays on Safari; None greys Volume.
   $("audio-source").addEventListener("change", () => { syncVolumeDisabled(); applyAudio(); debounced(); });
+  applyUrlAudioParams();        // shareable ?audio=/?vol= deep-link presets
   syncVolumeDisabled();
+  syncWelcomeVolumeDisabled();  // reflect a URL-set source on the welcome slider too
+  updateWelcomeAudioLabel();
   // Video toggle (panel): the FIRST time video turns on without having entered via
   // Launch, begin the experience too — couple audio in. (Normally Launch entered
   // first, so videoEverOn is already true and this is a no-op beyond re-render.)
diff --git a/simulator/static/credits.html b/simulator/static/credits.html
index ec973c5..aae3582 100644
--- a/simulator/static/credits.html
+++ b/simulator/static/credits.html
@@ -48,14 +48,18 @@
     <section>
       <h2>Sound</h2>
       <p class="note">
-        Soundtracks are public domain or CC0 — credit is courtesy, not required.
+        Per-altitude soundtracks are public domain or CC0 — credit is courtesy. The
+        <em>Music</em> option is Creative Commons <strong>BY</strong> — attribution required, given below.
       </p>
       <ul class="audio-credits">
         <li><strong>NASA / Chandra X-ray Observatory (CXC/SAO)</strong> — cosmos sonification (public domain).</li>
         <li><strong>NOAA</strong> SanctSound &amp; PMEL hydrophone recordings — reef/abyss biological layers (public domain).</li>
         <li><strong>freesound.org</strong> contributors — Sonicfreak (station room-tone) and DCSFX (underwater bed) (CC0).</li>
         <li><strong>BigSoundBank</strong> — sea waves &amp; gull calls (CC0).</li>
-        <li><strong>deadrobotmusic</strong> (freesound.org) — “Ambient F Sharp Minor Ethereal Choir Pad” (the <em>Music</em> option) (CC0).</li>
+        <li><strong>Kevin MacLeod</strong> (<a href="https://incompetech.com" rel="noopener">incompetech.com</a>) — the
+          <em>Music</em> option: “Music for Manatees”, “Dewdrop Fantasy”, “Eastminster”, “Overheat”, and
+          “Concentration”. Licensed under
+          <a href="https://creativecommons.org/licenses/by/4.0/" rel="license">Creative Commons: By Attribution 4.0</a>.</li>
       </ul>
     </section>
   </main>
diff --git a/simulator/static/i18n.js b/simulator/static/i18n.js
index b9d62ad..c7e1ab6 100644
--- a/simulator/static/i18n.js
+++ b/simulator/static/i18n.js
@@ -29,6 +29,7 @@
     "output.audio.music":      { en: "Music", es: "Música", fr: "Musique", ja: "音楽" },
     "output.volume":  { en: "Volume", es: "Volumen", fr: "Volume", ja: "音量" },
     "altitude.legend":{ en: "Altitude", es: "Altitud", fr: "Altitude", ja: "高度" },
+    "altitude.autoadvance": { en: "Auto-advance", es: "Avance automático", fr: "Avance automatique", ja: "自動で移動" },
     "knobs.legend":   { en: "Experience knobs", es: "Mandos de experiencia", fr: "Boutons d’expérience", ja: "体験つまみ" },
     "knobs.think":    { en: "Think", es: "Pensar", fr: "Penser", ja: "考える" },
     "knobs.feel":     { en: "Feel", es: "Sentir", fr: "Ressentir", ja: "感じる" },
diff --git a/simulator/static/index.html b/simulator/static/index.html
index c835324..9b7eec7 100644
--- a/simulator/static/index.html
+++ b/simulator/static/index.html
@@ -26,6 +26,13 @@
                  list="play-speed-ticks" aria-describedby="welcome-play-speed-val" />
           <span id="welcome-play-speed-val">1.00×</span>
         </label>
+        <label class="audio-source"><span data-i18n="output.audio">Audio</span>
+          <select id="welcome-audio-source" aria-label="Audio source">
+            <option value="none" data-i18n="output.audio.none">None</option>
+            <option value="soundtrack" data-i18n="output.audio.soundtrack" selected>Soundtrack</option>
+            <option value="music" data-i18n="output.audio.music">Music</option>
+          </select>
+        </label>
         <label class="audio-level"><span data-i18n="output.volume">Volume</span>
           <input type="range" id="welcome-audio" min="0" max="10" value="2" step="1" />
           <span id="welcome-audio-val">2</span>/10
@@ -73,6 +80,11 @@
                aria-valuemin="0" aria-valuenow="0" aria-valuetext="cosmos"></svg>
         </div>
         <span id="scale-name" class="scale-name">—</span>
+        <label class="dev-switch" for="auto-advance">
+          <input type="checkbox" id="auto-advance" checked />
+          <span class="dev-switch-track"><span class="dev-switch-thumb"></span></span>
+          <span class="dev-switch-label" data-i18n="altitude.autoadvance">Auto-advance</span>
+        </label>
       </fieldset>
 
       <fieldset>
diff --git a/simulator/static/scrub.js b/simulator/static/scrub.js
index 7ea930d..4dc7d83 100644
--- a/simulator/static/scrub.js
+++ b/simulator/static/scrub.js
@@ -44,6 +44,16 @@
     return { mode: "blend", lo: g.from * level, hi: g.to * level };
   }
 
+  // Pick the next track in a rotating playlist: a random index in [0,count) that is
+  // NEVER `current` (so a rotation always changes track), given r in [0,1). Maps r
+  // onto the (count-1) other slots. count<=1 stays on 0. Pure so the shuffle is
+  // node-testable; the caller passes Math.random().
+  function pickNextIndex(current, count, r) {
+    if (count <= 1) return 0;
+    const k = Math.min(count - 2, Math.floor(clamp01(r) * (count - 1)));
+    return k >= current ? k + 1 : k;
+  }
+
   // The base-loop frame a landing must continue from so the steady loop picks up
   // the EXACT frame the morph last showed — no jump. A morph spans src@0 (frac 0)
   // -> dst@loopTailS (frac 1), scrubbed bidirectionally. Descending (dir>0) lands
@@ -68,5 +78,5 @@
     return out;
   }
 
-  return { clamp01, wrapIndex, segmentOf, accumToPos, fracToTime, crossfadeGains, audioPlan, loopLandFrame, integerCrossings };
+  return { clamp01, wrapIndex, segmentOf, accumToPos, fracToTime, crossfadeGains, audioPlan, pickNextIndex, loopLandFrame, integerCrossings };
 });
diff --git a/simulator/static/style.css b/simulator/static/style.css
index 2bba2f2..7bc2081 100644
--- a/simulator/static/style.css
+++ b/simulator/static/style.css
@@ -97,7 +97,7 @@ main { display: flex; gap: 1rem; padding: 1rem; flex-wrap: wrap; align-items: fl
 .audio-status { margin-top: 0.5rem; font: 11px/1.4 monospace; color: #9ab; }
 .audio-status.playing { color: #4e9; }
 .audio-status.blocked { color: #f97; }
-.panel { flex: 0 0 280px; display: flex; flex-direction: column; gap: 0.8rem;
+.panel { flex: 0 0 300px; display: flex; flex-direction: column; gap: 0.8rem;
          max-height: calc(100vh - 2rem); overflow-y: auto;
          position: sticky; top: 1rem; }
 /* The control panel stays hidden behind the welcome screen and appears only once
@@ -151,7 +151,10 @@ input[type=range], select { width: 100%; }
 /* Audio level dial (0–10): label · slider · value on one row, matching the switch look. */
 .audio-level { display: flex; align-items: center; gap: 0.5rem; margin-top: 0.45rem;
                color: #9af; font-weight: 600; letter-spacing: 0.3px; }
-.audio-level input[type=range] { flex: 1; width: auto; }
+/* Keep the control label on one line ("Video speed" was wrapping); let the range
+   shrink (min-width:0) instead of pushing the label to a second row. */
+.audio-level > span:first-child { white-space: nowrap; }
+.audio-level input[type=range] { flex: 1; width: auto; min-width: 0; }
 #audio-level-val { font-variant-numeric: tabular-nums; min-width: 1.2em; text-align: right; }
 /* Playback speed (−2×…2×) reuses the audio-level row look; wider value field for the sign + decimals. */
 #play-speed-val, #welcome-play-speed-val { font-variant-numeric: tabular-nums; min-width: 3.4em; text-align: right; }
diff --git a/simulator/unit/audio.test.js b/simulator/unit/audio.test.js
index 10fa606..451c3bc 100644
--- a/simulator/unit/audio.test.js
+++ b/simulator/unit/audio.test.js
@@ -26,6 +26,26 @@ test("audioPlan: Music ignores altitude fraction (constant across the dial)", ()
   assert.deepEqual(a, b);
 });
 
+test("pickNextIndex: never returns the current track; covers the rest", () => {
+  // r in [0,1) maps onto the (count-1) other indices, skipping `current`.
+  assert.equal(S.pickNextIndex(0, 5, 0), 1);
+  assert.equal(S.pickNextIndex(0, 5, 0.999), 4);
+  assert.equal(S.pickNextIndex(2, 5, 0), 0);
+  assert.equal(S.pickNextIndex(2, 5, 0.5), 3);
+  assert.equal(S.pickNextIndex(4, 5, 0.999), 3);
+  // sweep: for each current, every r lands somewhere valid and never on current
+  for (let cur = 0; cur < 5; cur++) {
+    for (let i = 0; i < 20; i++) {
+      const n = S.pickNextIndex(cur, 5, i / 20);
+      assert.ok(n >= 0 && n < 5 && n !== cur, `cur=${cur} r=${i / 20} -> ${n}`);
+    }
+  }
+});
+
+test("pickNextIndex: single-track playlist stays on 0", () => {
+  assert.equal(S.pickNextIndex(0, 1, 0.7), 0);
+});
+
 test("audioPlan: Soundtrack blends lo/hi by fraction, scaled by level", () => {
   assert.deepEqual(S.audioPlan("soundtrack", 1, 0), { mode: "blend", lo: 1, hi: 0 });
   assert.deepEqual(S.audioPlan("soundtrack", 1, 1), { mode: "blend", lo: 0, hi: 1 });