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+# Audio "Music" option + "Volume" rename — design
+
+*Date: 2026-07-01 · Status: approved design, awaiting spec review*
+
+## Goal
+
+Give the listener a choice of what to hear. Today the Output panel has a single
+0–10 slider labeled **Audio** that acts as the master gain for the per-altitude
+ambience crossfade. We split that into two controls:
+
+- **Audio** — a dropdown selecting the audio *source*: **None · Soundtrack · Music**
+- **Volume** — the existing 0–10 slider (relabeled), master gain for whatever
+  source is active
+
+**Music** is one ethereal, relaxing ambient loop (the innerflo.me vibe) that
+plays continuously and does **not** change with altitude.
+
+## Non-goals
+
+- No per-altitude music set (Music is a single track, not a musical Soundtrack).
+- No change to the Soundtrack behavior itself (per-altitude crossfade stays exactly as-is).
+- No new audio elements — the existing `#aud` / `#aud-b` pair is reused.
+
+## Control surface
+
+The Output `<fieldset>` (`simulator/static/index.html`) gains a dropdown above the
+renamed slider:
+
+```
+Output
+  [ Video ]  (toggle)
+  Audio    [ Soundtrack ▾ ]     ← NEW: <select id="audio-source">
+  Volume   ●──────── 2/10        ← existing #audio slider, label → "Volume"
+  Speed    ●──────── 1.00×
+  🌐 [ Language ▾ ]
+```
+
+- The dropdown is a `<select id="audio-source">` with options `none` / `soundtrack`
+  / `music`, default **`soundtrack`** (preserves today's launch experience).
+- The slider keeps `id="audio"` (avoids touching every reference) but its visible
+  label becomes **Volume** (new i18n key `output.volume`). The `output.audio` key
+  ("Audio") now labels the dropdown.
+
+## Behavior by mode
+
+| Mode | Behavior |
+|---|---|
+| **None** | Both `#aud`/`#aud-b` fade to silence — the existing `audioLevel()===0` path. The Volume slider is disabled (greyed). |
+| **Soundtrack** | Unchanged: per-altitude ambience crossfading across the two elements as the dial moves (`blendAudio` / `restAudio`). |
+| **Music** | The single music loop plays on `#aud` at master volume; `#aud-b` fades out. Altitude changes do **not** crossfade or reload audio. |
+
+Volume = 0 remains a hard mute in every mode.
+
+## Code shape
+
+All in `simulator/static/app.js`, following the existing audio-layer seams
+(`restAudio`, `blendAudio`, `ensurePlaying`, `scaleAudioUrl`):
+
+- **`audioSource()`** — new helper reading `#audio-source` → `"none" | "soundtrack" | "music"`.
+- **`musicUrl()`** — returns the resolved music loop url via `mediaBase()`
+  (mirrors `scaleAudioUrl`'s R2/local routing), e.g. `mediaBase()+"audio/music/<name>.loop.mp3"`.
+- **`restAudio(index)`** branches:
+  - `none` (or `audioLevel()===0`) → existing fade-to-silence.
+  - `music` → `ensurePlaying(aud, musicUrl(), audioVol())`; `fadeVolume(audB, 0, …)`.
+  - `soundtrack` → unchanged per-altitude logic.
+- **`blendAudio(lo, frac)`** returns early (no crossfade) when source is `music` or
+  `none`; only Soundtrack blends across the altitude boundary.
+- **Dropdown wiring**: `#audio-source` `change` → reconcile synchronously in the
+  gesture (`applyAudio()`), same Safari-unlock discipline the slider already uses;
+  toggles the Volume slider's `disabled` when None.
+- **`beginExperience()`**: unchanged except it snaps **Volume** (`#audio`) to the
+  start level 2; default source stays Soundtrack.
+- **Diagnostic readout** (`audioStatusText`): prefix with the current source
+  (`none`/`soundtrack`/`music`) so a "no sound" report stays legible.
+
+No server/engine change — audio source is a pure client concern.
+
+## Music asset & licensing
+
+- One **CC0 / public-domain** ethereal ambient loop (innerflo.me-style: soft,
+  evolving, no percussion, seamless loop). Sourced by the implementer; reviewed by
+  ear before ship.
+- Stored at `simulator/.../media/audio/music/<name>.loop.mp3` alongside the
+  per-scale soundtracks, and registered in the credits manifest exactly like every
+  other audio asset (this project credits all CC/PD audio — see `credits.html`).
+- Fallback path constant mirrors the `SCALE_AUDIO_FALLBACK` pattern so Music works
+  even against an older server process.
+
+## i18n + a11y
+
+- New key `output.volume` (en: "Volume", es: "Volumen", fr: "Volume", ja: "音量").
+- Dropdown option labels localized in en/es/fr/ja:
+  - None (Ninguno / Aucun / なし)
+  - Soundtrack (Banda sonora / Bande-son / サウンドトラック)
+  - Music (Música / Musique / 音楽)
+- `<select id="audio-source">` gets an `aria-label` tied to the Audio label.
+- Reduced-motion behavior is unaffected (audio is orthogonal to motion).
+
+## Tests
+
+- **Node unit** (`simulator/unit/…`): `audioSource()` routing and the
+  `blendAudio`/`restAudio` branch selection — none→silent, music→single-element +
+  no crossfade on altitude change, soundtrack→unchanged.
+- **E2E** (`simulator/e2e/…`): dropdown switches modes; Music produces a single
+  playing element that persists across an altitude change; None disables the Volume
+  slider and silences audio; the slider label reads "Volume".
+
+## Decisions (confirmed with operator)
+
+1. **Music = one continuous, altitude-independent loop** (not per-altitude, not ducked).
+2. Default Audio source = **Soundtrack**; Music is opt-in.
+3. **None** disables the Volume slider.
+4. Music track is **operator-approved CC0** sourced by the implementer.