Comment 9 for bug 378470

Hm, I have the same problem, but independent of the mixer (I have been using gnome-volume-control for some time now, although I'm running Xubuntu), which suggests that the problem is not in xfce4-mixer, but in the alsa configuration, or pulseaudio itself.
I'll test the workaround suggested in bug #449783 on my next reboot.

This and all related bugs might in fact be duplicates of bug #449783.