Comment 13 for bug 136837

I just experienced this with an onboard intel card (82801G [ICH7 Family] High Definition Audio Controller). I get the random muting/unmuting when the applet is set to track (both by dragging slider and by mouse wheel but not when adjusting in mixer):

Headphone
Front
Front Mic
Line-in
CD
Mic
Aux-in
Capture

Only two options (PCM and iSpeaker) can be increased/decreased with no unexpected results.

If you would like any particular output let me know.