mpv's volume/mute properties are software volume applied before samples
reach the AO, and AVSampleBufferAudioRenderer queues ~2s ahead, so slider
and mute changes were heard only after the queue drained. With the
avfoundation AO active, drive ao-volume/ao-mute (renderer volume, instant)
and keep software volume neutral; other AOs keep software volume. Re-apply
on every audio reconfig since a new renderer starts at full volume.
cf1acb1986