Comment 39 for bug 878986

Kevin, please note the root cause is the communication between amixer and pulseaudio, see comment #29

command like "amixer -D pulse ..." could look like to "solve" this issue because amixer builds the mixer control directly from the "virtul" device Pulseaudio instead of the "real" device, say your audio card.

To bind the hotkey together with a new command like "amixer -D ..." is only a workaround.

PS
1. If you are interested in this, you could try "alsamixer" and "alsamixer -D pulse" and see what's difference between the device name on the alsamixer panel.

2. another interesting experiment: you could try to un-mute all of the channels besides Master (e.g. Speaker, Headphone, PCM ...etc. ), and you should see the sound comes back.