Try changing the Audio Midi settings and zapping the PRAM:
Edit: Apple's article on Audio Midi is gone. Audio Midi Setup in your Applications -> Utilities may need to be reset to 48kHz before some applications can play sound after an update.
In case this doesn't exactly work for you (it didn't for me) open Audio Midi Setup in Applications -> Utilities and note that you may need to de-select mute on each speaker, and reset the speaker sliders in the control panel. This is what worked for me on my mac mini quad i7 running OS 10.8.2. (Even though my top taskbar and keyboard shortcuts indicated my audio was on, my output was nevertheless set to mute in the Audio Midi Setup control panel.)
Thanks to a brody for telling me where to look.