docs: add music playback documentation

USAGE.md music section under Mumble, CHEATSHEET.md music commands,
TASKS.md sprint update for v2.3.0.

docs/USAGE.md

@@ -1553,6 +1553,36 @@ Mumble uses HTML for text messages. On receive, the bot strips tags and
 unescapes entities. On send, text is HTML-escaped. Action messages use
 `<i>` tags for italic formatting.
 
+### Music Playback
+
+Stream audio from YouTube, SoundCloud, and other yt-dlp-supported sites
+into the Mumble voice channel. Audio is decoded to PCM, encoded to Opus
+via system libopus, and transmitted as voice packets over the TCP tunnel.
+
+**System dependencies** (must be installed on the host):
+- `yt-dlp` -- audio stream extraction
+- `ffmpeg` -- decode to 48kHz mono s16le PCM
+- `libopus.so.0` -- Opus codec (ctypes, no Python binding)
+
+```
+!play <url>          Play audio or add to queue
+!stop                Stop playback, clear queue
+!skip                Skip current track
+!queue               Show queue
+!queue <url>         Add to queue (alias for !play)
+!np                  Now playing
+!volume [0-100]      Get/set volume
+```
+
+- Queue holds up to 50 tracks
+- Volume applies from the next track (default: 50%)
+- Title resolved via `yt-dlp --get-title` before playback
+- Audio pipeline: `yt-dlp | ffmpeg` subprocess, 20ms Opus frames
+- Commands are Mumble-only; `!play` on other adapters replies with an error,
+  other music commands silently no-op
+- Playback runs as an asyncio background task; the bot remains responsive
+  to text commands during streaming
+
 ### Transport
 
 TCP connections route through the SOCKS5 proxy at `127.0.0.1:1080`