I just upgraded my oldish MacBook Pro to Mountain Lion and I discovered that the sound output wasn't working. In the menu bar the speaker icon was faded out, and in the sound control panel the volume and mute controls were faded out too.
After some poking around on various Apple forums, I found the culprit: "coredaudiod". So I opened up the "Activity Monitor" application, selected the "coreaudiod" process in the list and hit "Quit Process" (picking "Force Quit" from the confirmation just to be extra sure).
It reappeared a few moments later, with all of the regular sound output restored. In fact, iTunes started playing my new Bend Sinister tracks instantly.