The only sound working properly is the power-beep. If there's music playing and I unplug my laptop then I can hear it for that half a second that the power beep makes a sound, so it seems that something is blocking my audio from getting to the speakers.
I had this problem with 12.04 as well.
I've reloaded and reinstalled alsa/pulseaudio. All the alsamixer/pavucontrols look fine to me.
It briefly worked, when I first switched from unity to gnome-shell and when I upgraded from 12.04 to 13.04, but now I have the same problem.