Comment 6 for bug 410047

This may help someone find the true cause of this bug, but I tried removing gstreamer0.10-pulseaudio and then installing gnome-alsa mixer and rebooting and the problem went away.

From here:
http://ubuntuforums.org/showthread.php?p=9365153

sudo apt-get remove gstreamer0.10-pulseaudio
sudo apt-get install gnome-alsamixer
reboot