Snow Leopard audio problem

I have an iMac with Snow Leopard running on it (factory installed).

If I leave the system on for a while, and come back to it, I have NO audio. I thought this was a fast user switching problem at first, but now it seems like the system loses sound after a couple hours or so.

Speakers are fine, volume is up, etc. But no sound.

Has anyone else seen this?

So far, the only way to fix it is reboot