No sound from Garageband, Ableton Live

Hey, I'm running Mac OS X Server 10.6.8. The sound works fine, except in these two applications. At no point has either delivered sound.

Garageband says "Error Code 28773 was returned by the Audio driver" if I touch the output settings, even to move it to the default. Ableton also just does not give sound, and if I touch the audio output, it says "Failed to open the audio device 'Built-in Output (0 in, 2 out).'" Ableton also mentioned a possible "corrupt audio driver" causing a crash.

Very few other programs have been installed, but this is my dad's old computer that had some medical practice software I deleted as thoroughly as I could see. Can anyone suggest what to look for? I've spent so long trying things.