Comment 3 for bug 901338

I had the same issue with my Logitech G35 headset (with internal USB sound card) volume controls actually adjusting the motherboard soundcard volume instead, but worked around the problem with these instructions:

http://ulno.net/2011/04/16/mixer-control-with-multimedia-keys-und-multiple-soundcards-in-xfce4-on-debian-or-ubuntu/

Probably nicer than using gnome-settings-daemon.