macOS transcode via afconvert; document ducking no-op

- convert_audio: use ffmpeg when present (Linux path unchanged); on macOS
  without ffmpeg, fall back to the built-in afconvert for m4a, with a clear
  "install ffmpeg for mp3/ogg/flac" error for the formats it can't encode.
- media_duck: document why macOS is left as a deliberate no-op — the only
  built-in volume control is system-wide and would dim our own afplay voice;
  proper per-app ducking needs CoreAudio, not osascript.

src/mcspeak/audio.py

@@ -94,13 +94,19 @@ class ConversionError(RuntimeError):
     """Raised when format conversion fails."""
 
 
+# macOS built-in afconvert can only encode a subset (AAC/m4a) — no mp3/ogg/flac
+# encoders ship with it. Those still need ffmpeg (brew install ffmpeg).
+_AFCONVERT_ARGS: dict[str, list[str]] = {
+    "m4a": ["-f", "m4af", "-d", "aac"],  # AAC in an MPEG-4 container
+}
+
+
 async def convert_audio(src_wav: Path, dest: Path, fmt: str) -> Path:
     """Convert a source WAV to the requested format at dest.
 
-    `fmt == "wav"` shells out to a plain copy (no ffmpeg invocation).
-    Other formats use ffmpeg with codec args from _FFMPEG_CODEC_ARGS.
-
-    Raises ConversionError on ffmpeg non-zero exit or unsupported format.
+    `fmt == "wav"` is a plain copy. Otherwise ffmpeg is used when available; on
+    macOS without ffmpeg, afconvert covers m4a (mp3/ogg/flac still need ffmpeg).
+    Raises ConversionError on failure or unsupported format.
     """
     if fmt not in SUPPORTED_FORMATS:
         raise ConversionError(
@@ -113,20 +119,44 @@ async def convert_audio(src_wav: Path, dest: Path, fmt: str) -> Path:
         shutil.copyfile(src_wav, dest)
         return dest
 
-    proc = await asyncio.create_subprocess_exec(
-        "ffmpeg", "-y", "-loglevel", "error", "-i", str(src_wav),
-        *_FFMPEG_CODEC_ARGS[fmt],
-        str(dest),
-        stdout=asyncio.subprocess.DEVNULL,
-        stderr=asyncio.subprocess.PIPE,
-    )
-    _, stderr = await proc.communicate()
-    if proc.returncode != 0:
-        raise ConversionError(
-            f"ffmpeg failed ({proc.returncode}) converting to {fmt}: "
-            f"{stderr.decode(errors='replace').strip()[:500]}"
+    if platform_audio.has_ffmpeg():
+        proc = await asyncio.create_subprocess_exec(
+            "ffmpeg", "-y", "-loglevel", "error", "-i", str(src_wav),
+            *_FFMPEG_CODEC_ARGS[fmt],
+            str(dest),
+            stdout=asyncio.subprocess.DEVNULL,
+            stderr=asyncio.subprocess.PIPE,
         )
-    return dest
+        _, stderr = await proc.communicate()
+        if proc.returncode != 0:
+            raise ConversionError(
+                f"ffmpeg failed ({proc.returncode}) converting to {fmt}: "
+                f"{stderr.decode(errors='replace').strip()[:500]}"
+            )
+        return dest
+
+    # No ffmpeg — fall back to the macOS built-in afconvert for what it can do.
+    if platform_audio.IS_MACOS:
+        args = _AFCONVERT_ARGS.get(fmt)
+        if args is None:
+            raise ConversionError(
+                f"macOS afconvert can't encode {fmt!r} (only m4a/wav without ffmpeg). "
+                f"Install ffmpeg (brew install ffmpeg) for mp3/ogg/flac."
+            )
+        proc = await asyncio.create_subprocess_exec(
+            "afconvert", *args, str(src_wav), str(dest),
+            stdout=asyncio.subprocess.DEVNULL,
+            stderr=asyncio.subprocess.PIPE,
+        )
+        _, stderr = await proc.communicate()
+        if proc.returncode != 0:
+            raise ConversionError(
+                f"afconvert failed ({proc.returncode}) converting to {fmt}: "
+                f"{stderr.decode(errors='replace').strip()[:500]}"
+            )
+        return dest
+
+    raise ConversionError(f"ffmpeg not found — needed to convert to {fmt!r}. Install ffmpeg.")
 
 
 class PlaybackError(RuntimeError):

src/mcspeak/media_duck.py

@@ -11,6 +11,13 @@ protocol connection, while pactl (using libpulse) works reliably.
 
 If pactl is not installed or the PulseAudio socket is unavailable, all
 operations silently no-op — TTS still works, just without ducking.
+
+macOS: intentionally left as that no-op. The only built-in volume control
+(`osascript ... set volume output volume`) is SYSTEM-WIDE, and our TTS plays
+through afplay as system audio — so ducking would dim our own voice, the
+opposite of the goal. Per-app ducking on macOS needs CoreAudio (not a
+built-in), so proper ducking is a future native-helper task, not an osascript
+one. Don't "fix" this by adding osascript volume control.
 """
 
 import asyncio