When using pass through audio via alsa muting does not work.
xineliboutput should just not send the audio track to xine if vdr is muted.
Have you tried different values for "Volume Control" in plugin Audio setup menu ?
I'm on vacation, I'll try when I'm back.
OK, for MP2 audio tracks muting works with software volume control.
But for AC3 audio tracks muting does not work, neither with software nor with hardware volume control.