diff --git a/player/audio.py b/player/audio.py
new file mode 100644
index 0000000..a2e706a
--- /dev/null
+++ b/player/audio.py
@@ -0,0 +1,54 @@
+"""Resolve the orthogonal Visual × Audio controls into render outputs (audio spec
+§4/§6). The single source of truth for: whether the projector shows video, and
+which audio url plays for a given (audio source, current altitude).
+
+Replaces player/content.py's 7-way bundled table. White-noise is a global,
+altitude-independent bed; soundtrack follows the single Altitude dial; off is
+silence. `music` is a reserved, deferred source — not in v1."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+VISUAL_POSITIONS = frozenset({"on", "off"})
+# v1 audio sources. "music" is reserved/deferred (no assets) and intentionally absent.
+AUDIO_SOURCES = frozenset({"off", "soundtrack", "white_noise"})
+
+
+@dataclass(frozen=True)
+class AudioResolution:
+    source: str
+    url: str | None
+    altitude_coupled: bool
+
+
+def resolve_visual(position: str) -> bool:
+    """Visual on/off → whether the renderer shows video (vs fade-to-black)."""
+    if position not in VISUAL_POSITIONS:
+        raise ValueError(
+            f"unknown visual position {position!r}; expected one of {sorted(VISUAL_POSITIONS)}"
+        )
+    return position == "on"
+
+
+def resolve_audio_url(source: str, *, scale_audio: str, noise_url: str) -> str | None:
+    """The audio url for (source, current altitude): soundtrack → the current
+    scale's asset (or None if none authored); white_noise → the global bed; off →
+    None. `scale_audio` is the ring scale's `audio` path (relative to /media/audio/)."""
+    if source not in AUDIO_SOURCES:
+        raise ValueError(
+            f"unknown audio source {source!r}; expected one of {sorted(AUDIO_SOURCES)}"
+        )
+    if source == "off":
+        return None
+    if source == "white_noise":
+        return noise_url
+    # soundtrack — couple to the current altitude's asset
+    return f"/media/audio/{scale_audio}" if scale_audio else None
+
+
+def resolve_audio(source: str, *, scale_audio: str, noise_url: str) -> AudioResolution:
+    """The full render.audio view: source, resolved url, and whether it follows the
+    Altitude dial (true only for soundtrack)."""
+    url = resolve_audio_url(source, scale_audio=scale_audio, noise_url=noise_url)
+    return AudioResolution(source=source, url=url, altitude_coupled=(source == "soundtrack"))
diff --git a/player/content.py b/player/content.py
deleted file mode 100644
index 035808d..0000000
--- a/player/content.py
+++ /dev/null
@@ -1,42 +0,0 @@
-"""Resolve the 7-way content dial into an audio source + video on/off (§6).
-
-The single source of truth for design §6's table: which audio source plays and
-whether the projector shows video, for each dial position.
-"""
-
-from __future__ import annotations
-
-from dataclasses import dataclass
-
-# The four distinct audio sources. "white_noise" is generated at runtime;
-# "music" is the public-domain classical pool; "audio_track" is the clip's own
-# field audio; "none" is silence.
-AUDIO_SOURCES = frozenset({"none", "white_noise", "music", "audio_track"})
-
-
-@dataclass(frozen=True)
-class ContentResolution:
-    audio_source: str
-    video: bool
-
-
-# Design §6, one row per dial position.
-_TABLE = {
-    "off": ContentResolution("none", False),
-    "white_noise": ContentResolution("white_noise", False),
-    "music": ContentResolution("music", False),
-    "audio_track": ContentResolution("audio_track", False),
-    "video": ContentResolution("none", True),
-    "music_video": ContentResolution("music", True),
-    "audio_video": ContentResolution("audio_track", True),
-}
-
-
-def resolve_content(position: str) -> ContentResolution:
-    """Map a content-dial position to its audio source and video flag (§6)."""
-    try:
-        return _TABLE[position]
-    except KeyError:
-        raise ValueError(
-            f"unknown content position {position!r}; expected one of {sorted(_TABLE)}"
-        ) from None
diff --git a/player/controls.py b/player/controls.py
index 9e66b43..49c43c8 100644
--- a/player/controls.py
+++ b/player/controls.py
@@ -1,20 +1,17 @@
 """Control-panel state: the data shape read from the Arduino over serial.
 
 This is the serial-contract payload shared with sub-project 4 (firmware). It
-models the full panel from design §6/§7: the 7-way content dial, the four
-experience knobs (0..4), and the two intensity levels (volume, brightness).
-The wire framing itself is the separate 3<->4 serial contract; this module is
-the *decoded* form.
+models the full panel from the audio spec §3: a Visual toggle (on/off) + an
+Audio source selector (off/soundtrack/white_noise), the four experience knobs
+(0..4), and the two intensity levels (volume, brightness). The wire framing
+itself is the separate 3<->4 serial contract; this module is the *decoded* form.
 """
 
 from __future__ import annotations
 
 from dataclasses import dataclass, fields
 
-# The seven positions of the content dial (design §6).
-CONTENT_POSITIONS = frozenset(
-    {"off", "white_noise", "music", "audio_track", "video", "music_video", "audio_video"}
-)
+from player.audio import AUDIO_SOURCES, VISUAL_POSITIONS
 
 KNOB_FIELDS = ("left", "right", "dark", "light", "volume", "brightness")
 KNOB_MIN = 0
@@ -27,7 +24,8 @@ class ControlsError(ValueError):
 
 @dataclass(frozen=True)
 class Controls:
-    content: str
+    visual: str
+    audio: str
     left: int
     right: int
     dark: int
@@ -38,10 +36,15 @@ class Controls:
 
 def validate_controls(c: Controls) -> None:
     """Raise ControlsError if the payload is structurally invalid."""
-    if c.content not in CONTENT_POSITIONS:
+    if c.visual not in VISUAL_POSITIONS:
         raise ControlsError(
-            f"invalid content position {c.content!r}; "
-            f"expected one of {sorted(CONTENT_POSITIONS)}"
+            f"invalid visual position {c.visual!r}; "
+            f"expected one of {sorted(VISUAL_POSITIONS)}"
+        )
+    if c.audio not in AUDIO_SOURCES:
+        raise ControlsError(
+            f"invalid audio source {c.audio!r}; "
+            f"expected one of {sorted(AUDIO_SOURCES)}"
         )
     for name in KNOB_FIELDS:
         value = getattr(c, name)
diff --git a/player/state.py b/player/state.py
index 85fad1e..d4a70b8 100644
--- a/player/state.py
+++ b/player/state.py
@@ -20,7 +20,7 @@ from typing import Callable, Optional
 
 from hef.selection import Coordinate
 from player.alteration import RenderPlan, plan_alteration
-from player.content import ContentResolution, resolve_content
+from player.audio import resolve_visual
 from player.controls import Controls
 
 
@@ -34,11 +34,14 @@ class TransitionKind:
 
 @dataclass(frozen=True)
 class Playback:
-    """What should currently be playing."""
+    """What should currently be playing. Visual and audio are orthogonal (audio
+    spec §2): `video` is whether the projector shows footage, `audio_source` is
+    the independent audio dial (off / soundtrack / white_noise)."""
 
     clip_id: Optional[str]            # None = black walls
     plan: Optional[RenderPlan]        # None when black
-    content: ContentResolution
+    video: bool                       # whether footage is shown (Visual on/off)
+    audio_source: str                 # the Audio dial source (off/soundtrack/white_noise)
     volume: int
     brightness: int
 
@@ -58,7 +61,8 @@ def _first(library):
 _BLACK = Playback(
     clip_id=None,
     plan=None,
-    content=ContentResolution("none", False),
+    video=False,
+    audio_source="off",
     volume=0,
     brightness=0,
 )
@@ -78,12 +82,13 @@ class Player:
         self._current = _BLACK
 
     def _resolve(self, controls: Controls) -> Playback:
-        content = resolve_content(controls.content)
-        if not content.video:
+        video = resolve_visual(controls.visual)
+        if not video:
             return Playback(
                 clip_id=None,
                 plan=None,
-                content=content,
+                video=False,
+                audio_source=controls.audio,
                 volume=controls.volume,
                 brightness=controls.brightness,
             )
@@ -92,7 +97,8 @@ class Player:
         return Playback(
             clip_id=clip.id,
             plan=plan_alteration(coord),
-            content=content,
+            video=True,
+            audio_source=controls.audio,
             volume=controls.volume,
             brightness=controls.brightness,
         )
diff --git a/simulator/app.py b/simulator/app.py
index 3f718b5..eaa8e6f 100644
--- a/simulator/app.py
+++ b/simulator/app.py
@@ -26,8 +26,7 @@ from player.alteration import (
     plan_alteration,
     render_plan_to_dict,
 )
-from player.content import resolve_content
-from player.controls import CONTENT_POSITIONS
+from player.audio import AUDIO_SOURCES, VISUAL_POSITIONS, resolve_audio, resolve_visual
 from player.ring import (
     DEFAULT_FAST_SPIN_THRESHOLD,
     advance_ring,
@@ -40,6 +39,9 @@ STATIC_DIR = Path(__file__).parent / "static"
 MEDIA_DIR = Path(__file__).parent / "sample_media"
 DEFAULT_MANIFEST = MEDIA_DIR / "manifest.json"
 
+# The global white-noise bed (audio spec §5.2): synthesized, altitude-independent.
+NOISE_URL = "/media/audio/noise/pink.mp3"
+
 # Per-process boot token: changes on every server restart so the dev live-reload
 # (below) also fires when Python code changes (which needs a restart), not just
 # when a static asset's mtime changes.
@@ -95,7 +97,8 @@ def _rev_file(file: str) -> str:
 
 
 class ControlsModel(BaseModel):
-    content: str
+    visual: str
+    audio: str
     left: int = Field(ge=0, le=4)
     right: int = Field(ge=0, le=4)
     dark: int = Field(ge=0, le=4)
@@ -112,6 +115,7 @@ class CalibrationModel(BaseModel):
 
 class AlterationRequest(BaseModel):
     controls: ControlsModel
+    altitude_index: int = 0
     calibration: Optional[CalibrationModel] = None
 
 
@@ -166,8 +170,10 @@ def create_app(manifest_path: Optional[Path] = None) -> FastAPI:
     @app.post("/api/alteration")
     def api_alteration(req: AlterationRequest):
         c = req.controls
-        if c.content not in CONTENT_POSITIONS:
-            raise HTTPException(status_code=422, detail=f"invalid content {c.content!r}")
+        if c.visual not in VISUAL_POSITIONS:
+            raise HTTPException(status_code=422, detail=f"invalid visual {c.visual!r}")
+        if c.audio not in AUDIO_SOURCES:
+            raise HTTPException(status_code=422, detail=f"invalid audio {c.audio!r}")
         coord = Coordinate(c.left, c.right, c.dark, c.light)
         cal = (
             Calibration(
@@ -179,10 +185,22 @@ def create_app(manifest_path: Optional[Path] = None) -> FastAPI:
             else DEFAULT_CALIBRATION
         )
         plan = plan_alteration(coord, cal)
-        content = resolve_content(c.content)
+        # Resolve the soundtrack url against the CURRENT altitude (server-side, audio
+        # spec §6.1). White-noise/off ignore the scale; soundtrack follows it.
+        scale_audio = ""
+        if app.state.ring is not None:
+            scale_audio = scale_at(app.state.ring, req.altitude_index).audio
+        audio = resolve_audio(c.audio, scale_audio=scale_audio, noise_url=NOISE_URL)
         return {
             "plan": render_plan_to_dict(plan),
-            "content": {"audio_source": content.audio_source, "video": content.video},
+            "render": {
+                "video": {"shown": resolve_visual(c.visual)},
+                "audio": {
+                    "source": audio.source,
+                    "url": audio.url,
+                    "altitude_coupled": audio.altitude_coupled,
+                },
+            },
         }
 
     @app.get("/dev/version")
diff --git a/tests/test_player_audio.py b/tests/test_player_audio.py
new file mode 100644
index 0000000..c65bc68
--- /dev/null
+++ b/tests/test_player_audio.py
@@ -0,0 +1,63 @@
+"""Unit tests for the pure Visual×Audio resolver (audio spec §4/§6)."""
+import pytest
+
+from player.audio import (
+    AUDIO_SOURCES,
+    VISUAL_POSITIONS,
+    AudioResolution,
+    resolve_audio,
+    resolve_audio_url,
+    resolve_visual,
+)
+
+NOISE = "/media/audio/noise/pink.mp3"
+
+
+def test_visual_on_off():
+    assert resolve_visual("on") is True
+    assert resolve_visual("off") is False
+
+
+def test_visual_rejects_unknown():
+    with pytest.raises(ValueError):
+        resolve_visual("flicker")
+
+
+def test_visual_positions_are_on_off():
+    assert VISUAL_POSITIONS == frozenset({"on", "off"})
+
+
+def test_soundtrack_resolves_to_the_current_scales_asset():
+    url = resolve_audio_url("soundtrack", scale_audio="coast/waves.loop.mp3", noise_url=NOISE)
+    assert url == "/media/audio/coast/waves.loop.mp3"
+
+
+def test_white_noise_resolves_to_the_global_bed_independent_of_altitude():
+    url = resolve_audio_url("white_noise", scale_audio="coast/waves.loop.mp3", noise_url=NOISE)
+    assert url == NOISE
+
+
+def test_off_resolves_to_silence():
+    assert resolve_audio_url("off", scale_audio="coast/waves.loop.mp3", noise_url=NOISE) is None
+
+
+def test_soundtrack_with_no_scale_asset_is_silent():
+    assert resolve_audio_url("soundtrack", scale_audio="", noise_url=NOISE) is None
+
+
+def test_resolve_audio_marks_only_soundtrack_altitude_coupled():
+    r = resolve_audio("soundtrack", scale_audio="reef/soundscape.loop.mp3", noise_url=NOISE)
+    assert isinstance(r, AudioResolution)
+    assert r.source == "soundtrack" and r.altitude_coupled is True
+    assert r.url == "/media/audio/reef/soundscape.loop.mp3"
+    assert resolve_audio("white_noise", scale_audio="x/y.mp3", noise_url=NOISE).altitude_coupled is False
+    assert resolve_audio("off", scale_audio="x/y.mp3", noise_url=NOISE).altitude_coupled is False
+
+
+def test_resolve_audio_url_rejects_unknown_source():
+    with pytest.raises(ValueError):
+        resolve_audio_url("podcast", scale_audio="", noise_url=NOISE)
+
+
+def test_music_is_not_a_v1_source():
+    assert "music" not in AUDIO_SOURCES
diff --git a/tests/test_player_content.py b/tests/test_player_content.py
deleted file mode 100644
index 70b3d4e..0000000
--- a/tests/test_player_content.py
+++ /dev/null
@@ -1,34 +0,0 @@
-import pytest
-
-from player.content import AUDIO_SOURCES, ContentResolution, resolve_content
-
-
-def test_audio_sources_are_the_four_distinct_sources():
-    assert AUDIO_SOURCES == frozenset({"none", "white_noise", "music", "audio_track"})
-
-
-# The §6 table, row by row: position -> (audio_source, video).
-@pytest.mark.parametrize(
-    "position,audio_source,video",
-    [
-        ("off", "none", False),
-        ("white_noise", "white_noise", False),
-        ("music", "music", False),
-        ("audio_track", "audio_track", False),
-        ("video", "none", True),
-        ("music_video", "music", True),
-        ("audio_video", "audio_track", True),
-    ],
-)
-def test_resolve_content_matches_spec_table(position, audio_source, video):
-    assert resolve_content(position) == ContentResolution(audio_source=audio_source, video=video)
-
-
-def test_off_is_void_state_black_and_silent():
-    r = resolve_content("off")
-    assert r.video is False and r.audio_source == "none"
-
-
-def test_resolve_content_rejects_unknown_position():
-    with pytest.raises(ValueError):
-        resolve_content("bogus")
diff --git a/tests/test_player_controls.py b/tests/test_player_controls.py
index 1c9f910..794723f 100644
--- a/tests/test_player_controls.py
+++ b/tests/test_player_controls.py
@@ -1,27 +1,36 @@
 import pytest
 
+from player.audio import AUDIO_SOURCES, VISUAL_POSITIONS
 from player.controls import (
     Controls,
-    CONTENT_POSITIONS,
     ControlsError,
     validate_controls,
     parse_controls,
 )
 
 
-def test_content_positions_are_the_seven_from_the_spec():
-    assert CONTENT_POSITIONS == frozenset(
-        {"off", "white_noise", "music", "audio_track", "video", "music_video", "audio_video"}
-    )
+def test_visual_and_audio_enums_are_the_v1_positions():
+    assert VISUAL_POSITIONS == frozenset({"on", "off"})
+    assert AUDIO_SOURCES == frozenset({"off", "soundtrack", "white_noise"})
 
 
 def test_valid_controls_pass_validation():
-    c = Controls(content="video", left=0, right=4, dark=2, light=2, volume=3, brightness=4)
+    c = Controls(visual="on", audio="soundtrack", left=0, right=4, dark=2, light=2,
+                 volume=3, brightness=4)
     validate_controls(c)  # does not raise
 
 
-def test_invalid_content_position_rejected():
-    c = Controls(content="bogus", left=0, right=0, dark=0, light=0, volume=0, brightness=0)
+def test_invalid_visual_position_rejected():
+    c = Controls(visual="flicker", audio="off", left=0, right=0, dark=0, light=0,
+                 volume=0, brightness=0)
+    with pytest.raises(ControlsError):
+        validate_controls(c)
+
+
+def test_invalid_audio_source_rejected():
+    # "music" is a reserved/deferred dial position, not a v1 source
+    c = Controls(visual="on", audio="music", left=0, right=0, dark=0, light=0,
+                 volume=0, brightness=0)
     with pytest.raises(ControlsError):
         validate_controls(c)
 
@@ -29,7 +38,8 @@ def test_invalid_content_position_rejected():
 @pytest.mark.parametrize("field", ["left", "right", "dark", "light", "volume", "brightness"])
 @pytest.mark.parametrize("bad", [-1, 5, True])
 def test_out_of_range_or_non_int_knob_rejected(field, bad):
-    kwargs = dict(content="off", left=0, right=0, dark=0, light=0, volume=0, brightness=0)
+    kwargs = dict(visual="off", audio="off", left=0, right=0, dark=0, light=0,
+                  volume=0, brightness=0)
     kwargs[field] = bad
     with pytest.raises(ControlsError):
         validate_controls(Controls(**kwargs))
@@ -37,17 +47,18 @@ def test_out_of_range_or_non_int_knob_rejected(field, bad):
 
 def test_parse_controls_from_mapping():
     c = parse_controls(
-        {"content": "music_video", "left": 1, "right": 2, "dark": 3, "light": 0, "volume": 2, "brightness": 1}
+        {"visual": "on", "audio": "white_noise", "left": 1, "right": 2, "dark": 3,
+         "light": 0, "volume": 2, "brightness": 1}
     )
-    assert c == Controls("music_video", 1, 2, 3, 0, 2, 1)
+    assert c == Controls("on", "white_noise", 1, 2, 3, 0, 2, 1)
 
 
 def test_parse_controls_rejects_unknown_keys():
     with pytest.raises(ControlsError):
-        parse_controls({"content": "off", "left": 0, "right": 0, "dark": 0,
+        parse_controls({"visual": "off", "audio": "off", "left": 0, "right": 0, "dark": 0,
                         "light": 0, "volume": 0, "brightness": 0, "bogus": 1})
 
 
 def test_parse_controls_rejects_missing_keys():
     with pytest.raises(ControlsError):
-        parse_controls({"content": "off", "left": 0})
+        parse_controls({"visual": "off", "left": 0})
diff --git a/tests/test_player_state.py b/tests/test_player_state.py
index 9611da6..5072352 100644
--- a/tests/test_player_state.py
+++ b/tests/test_player_state.py
@@ -14,47 +14,57 @@ class FakeClip:
 LIB = [FakeClip("base-a"), FakeClip("base-b")]
 
 
-def _controls(content="video", left=0, right=0, dark=0, light=0, volume=2, brightness=2):
-    return Controls(content, left, right, dark, light, volume, brightness)
+def _controls(visual="on", audio="off", left=0, right=0, dark=0, light=0,
+              volume=2, brightness=2):
+    return Controls(visual, audio, left, right, dark, light, volume, brightness)
 
 
 def test_first_update_to_video_fades_in_from_black():
     p = Player(LIB)
-    t = p.update(_controls(content="video"))
+    t = p.update(_controls(visual="on"))
     assert t.kind == TransitionKind.FADE_FROM_BLACK
     assert t.playback.clip_id == "base-a"
-    assert t.playback.content.video is True
+    assert t.playback.video is True
 
 
-def test_off_from_video_fades_to_black_and_silences():
+def test_visual_off_from_video_fades_to_black():
     p = Player(LIB)
-    p.update(_controls(content="video"))
-    t = p.update(_controls(content="off"))
+    p.update(_controls(visual="on"))
+    t = p.update(_controls(visual="off"))
     assert t.kind == TransitionKind.FADE_TO_BLACK
     assert t.playback.clip_id is None
-    assert t.playback.content.audio_source == "none"
+    assert t.playback.video is False
+    assert t.playback.audio_source == "off"
+
+
+def test_audio_survives_visual_off():
+    # white noise + black is now reachable (the gap the bundled dial omitted)
+    p = Player(LIB)
+    t = p.update(_controls(visual="off", audio="white_noise"))
+    assert t.playback.video is False
+    assert t.playback.audio_source == "white_noise"
 
 
 def test_no_change_yields_none_transition():
     p = Player(LIB)
-    p.update(_controls(content="video", left=1))
-    t = p.update(_controls(content="video", left=1))
+    p.update(_controls(visual="on", left=1))
+    t = p.update(_controls(visual="on", left=1))
     assert t.kind == TransitionKind.NONE
 
 
 def test_grade_change_is_a_live_update_not_a_crossfade():
     # design §4.3: the Dark/Light grade is a continuous runtime op
     p = Player(LIB)
-    p.update(_controls(content="video", dark=0, light=0))
-    t = p.update(_controls(content="video", dark=4, light=0))
+    p.update(_controls(visual="on", dark=0, light=0))
+    t = p.update(_controls(visual="on", dark=4, light=0))
     assert t.kind == TransitionKind.LIVE_UPDATE
     assert t.playback.plan.grade.tone == -1.0
 
 
 def test_overlay_change_is_a_live_update():
     p = Player(LIB)
-    p.update(_controls(content="video", left=0))
-    t = p.update(_controls(content="video", left=4))
+    p.update(_controls(visual="on", left=0))
+    t = p.update(_controls(visual="on", left=4))
     assert t.kind == TransitionKind.LIVE_UPDATE
     assert t.playback.plan.overlay.level == 4
 
@@ -63,8 +73,8 @@ def test_dream_change_is_a_live_update_not_a_crossfade():
     # Right-axis dream reframe (session 0013): the Right dream is a deterministic
     # LIVE filter now, so changing it no longer crossfades — only a clip swap does.
     p = Player(LIB)
-    p.update(_controls(content="video", right=0))
-    t = p.update(_controls(content="video", right=4))
+    p.update(_controls(visual="on", right=0))
+    t = p.update(_controls(visual="on", right=4))
     assert t.kind == TransitionKind.LIVE_UPDATE
     assert t.playback.plan.dream.strength == 4
     assert t.playback.plan.dream.intensity == 1.0
@@ -72,43 +82,43 @@ def test_dream_change_is_a_live_update_not_a_crossfade():
 
 def test_volume_only_change_is_a_live_update():
     p = Player(LIB)
-    p.update(_controls(content="video", volume=1))
-    t = p.update(_controls(content="video", volume=4))
+    p.update(_controls(visual="on", volume=1))
+    t = p.update(_controls(visual="on", volume=4))
     assert t.kind == TransitionKind.LIVE_UPDATE
     assert t.playback.volume == 4
 
 
 def test_audio_source_change_while_black_is_a_live_update():
     p = Player(LIB)
-    p.update(_controls(content="white_noise"))
-    t = p.update(_controls(content="music"))
+    p.update(_controls(visual="off", audio="white_noise"))
+    t = p.update(_controls(visual="off", audio="soundtrack"))
     assert t.kind == TransitionKind.LIVE_UPDATE
-    assert t.playback.content.audio_source == "music"
+    assert t.playback.audio_source == "soundtrack"
 
 
 def test_injected_base_chooser_is_used():
     p = Player(LIB, choose_base=lambda lib: lib[1])
-    t = p.update(_controls(content="video"))
+    t = p.update(_controls(visual="on"))
     assert t.playback.clip_id == "base-b"
 
 
 def test_empty_library_with_video_raises():
     p = Player([])
     with pytest.raises(ValueError):
-        p.update(_controls(content="video"))
+        p.update(_controls(visual="on"))
 
 
 def test_off_with_empty_library_is_fine():
     p = Player([])
     # levels at 0 match the initial black state, so this is a no-op transition
-    t = p.update(_controls(content="off", volume=0, brightness=0))
+    t = p.update(_controls(visual="off", volume=0, brightness=0))
     assert t.kind == TransitionKind.NONE  # already black at init
     assert t.playback.clip_id is None
 
 
 def test_off_with_levels_from_black_is_a_live_update():
     p = Player([])
-    t = p.update(_controls(content="off", volume=3, brightness=2))
+    t = p.update(_controls(visual="off", volume=3, brightness=2))
     assert t.kind == TransitionKind.LIVE_UPDATE  # black->black, levels set
     assert t.playback.clip_id is None
     assert t.playback.volume == 3
diff --git a/tests/test_simulator_api.py b/tests/test_simulator_api.py
index 94f7330..14f094a 100644
--- a/tests/test_simulator_api.py
+++ b/tests/test_simulator_api.py
@@ -28,8 +28,9 @@ def client(manifest_path):
     return TestClient(create_app(manifest_path=manifest_path))
 
 
-def _controls(content="video", left=0, right=0, dark=0, light=0, volume=2, brightness=2):
-    return dict(content=content, left=left, right=right, dark=dark,
+def _controls(visual="on", audio="off", left=0, right=0, dark=0, light=0,
+              volume=2, brightness=2):
+    return dict(visual=visual, audio=audio, left=left, right=right, dark=dark,
                 light=light, volume=volume, brightness=brightness)
 
 
@@ -41,13 +42,42 @@ def test_alteration_returns_the_engine_plan(client):
     assert data["plan"]["dream"]["strength"] == 2
     assert data["plan"]["dream"]["intensity"] == 0.5
     assert data["plan"]["grade"]["tone"] == -1.0
-    assert data["content"]["video"] is True
+    assert data["render"]["video"]["shown"] is True
 
 
-def test_alteration_honors_off_as_black(client):
-    resp = client.post("/api/alteration", json={"controls": _controls(content="off")})
-    data = resp.json()
-    assert data["content"]["video"] is False
+def test_alteration_returns_render_video_and_audio(client):
+    body = client.post("/api/alteration", json={"controls": _controls()}).json()
+    assert body["render"]["video"]["shown"] is True
+    assert body["render"]["audio"] == {"source": "off", "url": None, "altitude_coupled": False}
+
+
+def test_alteration_visual_off_hides_video(client):
+    body = client.post("/api/alteration", json={"controls": _controls(visual="off")}).json()
+    assert body["render"]["video"]["shown"] is False
+
+
+def test_alteration_white_noise_is_altitude_independent(ring_client):
+    for idx in (0, 1, 2):
+        body = ring_client.post("/api/alteration", json={
+            "controls": _controls(audio="white_noise"), "altitude_index": idx,
+        }).json()
+        assert body["render"]["audio"]["url"] == "/media/audio/noise/pink.mp3"
+        assert body["render"]["audio"]["altitude_coupled"] is False
+
+
+def test_alteration_soundtrack_couples_to_the_given_altitude(ring_client):
+    # ring fixture order: cosmos(0), forest(1), abyss(2) — each carries its audio
+    body = ring_client.post("/api/alteration", json={
+        "controls": _controls(audio="soundtrack"), "altitude_index": 0,
+    }).json()
+    a = body["render"]["audio"]
+    assert a["source"] == "soundtrack" and a["altitude_coupled"] is True
+    assert a["url"] == "/media/audio/cosmos/pillars.loop.mp3"
+    # a different altitude resolves a different soundtrack url
+    body2 = ring_client.post("/api/alteration", json={
+        "controls": _controls(audio="soundtrack"), "altitude_index": 2,
+    }).json()
+    assert body2["render"]["audio"]["url"] == "/media/audio/abyss/whale.loop.mp3"
 
 
 def test_alteration_accepts_calibration(client):
@@ -62,8 +92,13 @@ def test_alteration_rejects_out_of_range_knob(client):
     assert resp.status_code == 422
 
 
-def test_alteration_rejects_bad_content(client):
-    resp = client.post("/api/alteration", json={"controls": _controls(content="banana")})
+def test_alteration_rejects_bad_audio(client):
+    resp = client.post("/api/alteration", json={"controls": _controls(audio="music")})
+    assert resp.status_code == 422
+
+
+def test_alteration_rejects_bad_visual(client):
+    resp = client.post("/api/alteration", json={"controls": _controls(visual="dim")})
     assert resp.status_code == 422