diff --git a/simulator/static/app.js b/simulator/static/app.js
index 28bd42b..e08f09b 100644
--- a/simulator/static/app.js
+++ b/simulator/static/app.js
@@ -96,7 +96,10 @@ let mediaVersions = {};   // file -> content-hash token (from /api/media-version
 // changes with its bytes (permanent cache-bust). The blob cache, when present,
 // wins — those bytes are already the current ones for this session.
 function mediaNetUrl(file) { const v = mediaVersions[file]; return "/media/" + file + (v ? "?v=" + v : ""); }
-function mediaUrl(file) { return mediaBlobs[file] || mediaNetUrl(file); }
+function mediaUrl(file) {
+  if (file.startsWith("/media/")) return file;   // already a resolved absolute url (audio layer)
+  return mediaBlobs[file] || mediaNetUrl(file);
+}
 
 // Every media file the ring can show: each scale-pool clip's base footage plus the
 // per-edge transition clips. This is the full preload set (~two dozen files).
@@ -492,12 +495,14 @@ function renderScaleReadout() {
 }
 
 function controls() {
-  // One bipolar Mood dial (-4 dark .. 0 neutral .. +4 light) maps onto the engine's
-  // two poles. The calibration gains are pinned to their locked 1.0 (full dial = full
-  // effect), so the simulator sends no calibration — the server uses DEFAULT_CALIBRATION.
+  // Visual (on/off) and Audio (off/soundtrack/white_noise) are orthogonal dials
+  // (audio spec §2). One bipolar Mood dial (-4 dark .. +4 light) maps onto the
+  // engine's two poles. Calibration gains are pinned to 1.0, so the simulator
+  // sends no calibration — the server uses DEFAULT_CALIBRATION.
   const mood = +$("mood").value;
   return {
-    content: $("content").value,
+    visual: $("visual").checked ? "on" : "off",
+    audio: $("audio").value,
     left: +$("left").value, right: +$("right").value,
     dark: mood < 0 ? -mood : 0, light: mood > 0 ? mood : 0,
     volume: 2, brightness: 2,
@@ -509,12 +514,13 @@ async function update() {
   if (busy) return;
   const resp = await fetch("/api/alteration", {
     method: "POST", headers: { "content-type": "application/json" },
-    body: JSON.stringify({ controls: controls() }),
+    body: JSON.stringify({ controls: controls(), altitude_index: ringIndex }),
   });
   if (!resp.ok) { readout.textContent = "invalid: " + resp.status; return; }
   const data = await resp.json();
   readout.textContent = JSON.stringify(data, null, 2);
-  if (!data.content.video) { black.style.opacity = "1"; black.classList.remove("hidden"); return; }
+  applyAudio(data.render.audio);                       // reconcile the audio layer
+  if (!data.render.video.shown) { black.style.opacity = "1"; black.classList.remove("hidden"); return; }
   black.classList.add("hidden");
   try {
     ensureClipMedia();
@@ -894,6 +900,66 @@ function devLiveReload() {
   }, 1000);
 }
 
+// --- Audio layer: two <audio> elements, gain-crossfaded (audio spec §7/§8) ---
+const audA = $("audA"), audB = $("audB");
+let audActive = audA, audIdle = audB;   // which element is currently audible
+let audUrl = null;                      // the url currently playing (null = silence)
+let audioReady = false;                 // unlocked after the first user gesture (autoplay policy)
+const XFADE_MS = 600;                   // ~0.6s gain crossfade — "live", no busy ack (§7)
+
+function fadeVolume(el, to, ms) {
+  const from = el.volume, t0 = performance.now();
+  return new Promise((resolve) => {
+    function tick(now) {
+      const k = Math.min(1, (now - t0) / ms);
+      el.volume = from + (to - from) * k;
+      if (k < 1) requestAnimationFrame(tick);
+      else resolve();
+    }
+    requestAnimationFrame(tick);
+  });
+}
+
+// Reconcile the audio layer to render.audio: crossfade the active element to the
+// new url (or fade to silence for off). White-noise and soundtrack are the same
+// mechanism — only the url differs; an unchanged url is a no-op so altitude
+// re-rolls on the same scale don't restart the bed.
+async function applyAudio(audio) {
+  const url = audio ? audio.url : null;
+  if (url === audUrl) return;
+  audUrl = url;
+  if (!audioReady) return;              // deferred until the start gesture (§8)
+  if (!url) {                           // fade current out to silence
+    await fadeVolume(audActive, 0, XFADE_MS);
+    audActive.pause();
+    return;
+  }
+  audIdle.src = mediaUrl(url);          // absolute /media/... passes through mediaUrl
+  audIdle.volume = 0;
+  await audIdle.play().catch(() => {});
+  await Promise.all([fadeVolume(audIdle, 1, XFADE_MS), fadeVolume(audActive, 0, XFADE_MS)]);
+  audActive.pause();
+  [audActive, audIdle] = [audIdle, audActive];   // swap roles
+}
+
+// Browsers block autoplay until a user gesture. The renderer is launched
+// full-screen by the operator; a one-time tap unlocks audio, then it follows
+// state (audio spec §8 — the documented launch step).
+function showStartGesture() {
+  const ov = document.createElement("div");
+  ov.id = "start-gesture";
+  ov.textContent = "▶ tap to start sound";
+  ov.addEventListener("click", async () => {
+    audioReady = true;
+    // prime both elements within the gesture so later play() calls are allowed
+    for (const el of [audA, audB]) { try { await el.play(); el.pause(); } catch (_) {} }
+    ov.remove();
+    audUrl = null;                       // force the next applyAudio to (re)load
+    update();                            // re-apply current state, now with audio
+  }, { once: true });
+  document.body.appendChild(ov);
+}
+
 async function main() {
   devLiveReload();
   try { initPaint(); } catch (e) { paintOK = false; paint.style.display = "none"; showError("WebGL init: " + e.message); }
@@ -903,9 +969,10 @@ async function main() {
   buildDial();                // draw the altitude knob from the ring's scales
   initDev();                  // wire the Dev Mode toggle + pool picker (reads persisted state)
   renderScaleReadout();
-  for (const id of ["content", "left", "right", "mood"]) {
+  for (const id of ["visual", "audio", "left", "right", "mood"]) {
     $(id).addEventListener("input", debounced);
   }
+  showStartGesture();         // one-time tap unlocks audio (browser autoplay policy, §8)
   // Altitude knob: drag to turn (commit detents on release), scroll to step, tap a label to jump.
   dial.addEventListener("pointerdown", onDialDown);
   window.addEventListener("pointermove", onDialMove);
diff --git a/simulator/static/index.html b/simulator/static/index.html
index 46a2831..4582ef8 100644
--- a/simulator/static/index.html
+++ b/simulator/static/index.html
@@ -12,6 +12,8 @@
     <section class="stage" id="stage">
       <div class="screen">
         <video id="vid" loop muted playsinline></video>
+        <audio id="audA" loop preload="auto"></audio>
+        <audio id="audB" loop preload="auto"></audio>
         <canvas id="paint"></canvas>
         <div id="tint"></div>
         <svg id="overlay" viewBox="0 0 100 100" preserveAspectRatio="none"></svg>
@@ -22,15 +24,16 @@
 
     <section class="panel">
       <fieldset>
-        <legend>Content dial</legend>
-        <select id="content">
-          <option value="video">video</option>
-          <option value="audio_video">audio + video</option>
-          <option value="music_video">music + video</option>
-          <option value="off">off (black)</option>
-          <option value="white_noise">white noise (no video)</option>
-          <option value="music">music (no video)</option>
-          <option value="audio_track">audio track (no video)</option>
+        <legend>Visual</legend>
+        <label class="toggle"><input type="checkbox" id="visual" checked /> show video</label>
+      </fieldset>
+
+      <fieldset>
+        <legend>Audio</legend>
+        <select id="audio">
+          <option value="off">off (silence)</option>
+          <option value="soundtrack">soundtrack (follows altitude)</option>
+          <option value="white_noise">white noise</option>
         </select>
       </fieldset>
 
diff --git a/simulator/static/style.css b/simulator/static/style.css
index dc195a5..e615760 100644
--- a/simulator/static/style.css
+++ b/simulator/static/style.css
@@ -107,3 +107,13 @@ input[type=range], select { width: 100%; }
 .dev-anno .anno-key.measure { color: #ffd79a; }
 .dev-anno .anno-meta { color: #678; }
 .dev-anno .anno-empty { color: #567; font-style: italic; }
+
+/* One-time tap-to-start overlay — unlocks audio under the browser autoplay
+   policy (audio spec §8). Removed after the first gesture. */
+#start-gesture {
+  position: fixed; inset: 0; z-index: 10000;
+  display: flex; align-items: center; justify-content: center;
+  background: rgba(0, 0, 0, 0.82); color: #fff;
+  font: 600 28px/1.2 system-ui, sans-serif; letter-spacing: 0.02em;
+  cursor: pointer; user-select: none;
+}