docs(media): correct the audio claim — a video file's sound is silent, not loopback-carried

docs/ai/frame-pipeline.md

@@ -799,8 +799,9 @@ Full suite 290/291 passing, the sole failure the pre-existing compositor pixel t
   released (`GetActiveKeys`). Loop is a manager-level per-path answer: the ctor takes
   `Func<string,bool>? loopProvider` (production: "any live media reference loops"), applied before
   `StartAsync`, and `SetLooping(key, bool)` applies a live toggle. A file swap releases the old path after
-  the layer no longer references it. Volume stays unwired on purpose — sound rides the desktop loopback
-  (TRAX-style), so a per-file slider would be a control with no effect.
+  the layer no longer references it. **A video file's own soundtrack is silent today** — the decode pipe
+  is `-an`, so nothing plays it and the loopback has nothing to carry; wiring it (alert-style mixer feed
+  or TRAX-style local playback) is open, and no volume slider exists until then.
 - **Social bar on the output (bar bug-fix branch):** the `FramePump` takes an optional
   `socialBar: Func<(VideoFrame? Frame, SocialBarPosition Position)>?` seam, re-read **every frame** (so a
   mid-stream position flip applies immediately). The strip is pre-rasterized by `Compositor/SocialBarRenderer.cs`