feat: add sound listener hook to MumbleBot

Allow plugins to register callbacks for incoming voice PCM via
bot._sound_listeners. Empty list by default = zero overhead.

src/derp/mumble.py

@@ -158,6 +158,7 @@ class MumbleBot:
         self._loop: asyncio.AbstractEventLoop | None = None
         self._last_voice_ts: float = 0.0
         self._connect_count: int = 0
+        self._sound_listeners: list = []
 
         rate_cfg = config.get("bot", {})
         self._bucket = _TokenBucket(
@@ -235,6 +236,11 @@ class MumbleBot:
         if prev == 0.0:
             name = user["name"] if isinstance(user, dict) else "?"
             log.info("mumble: first voice packet from %s", name)
+        for fn in self._sound_listeners:
+            try:
+                fn(user, sound_chunk)
+            except Exception:
+                log.exception("mumble: sound listener error")
 
     def _on_text_message(self, message) -> None:
         """Callback from pymumble thread: text message received.