Sometimes I have issues with audio on my Ubuntu installation, usually after some rogue Flash applet destroys it (don’t ask me why.) What works for me is the following:
$ pulseaudio -k
After I do that pulseaudio restarts itself, but in case there is no audio you can try to followup with:
$ pulseaudio
Use at your own risk (I guess the worst that can happen is that you’ll have to reboot)