iTunes won't play, and no sound from any application

I have a troubling problem: when I press the play button on iTunes, the file does not play. The play symbol changes to the pause symbol, as if it was playing, but the cursor on the time bar does not move to the right and there is no sound. The problem is actually bigger than that. My sound does not work period. It's not just that sound doesn't come out of the speakers, but the volume keys don't actually do anything (the increase or decrease volume image does not come up on the screen). So I don't know if it's just a sound problem, or some bigger global problem, but it definitely is more than just a faulty speaker connection.

I'm using an eMac, OS 10.2.8,700 MHz PowerPC G4, 768 MB memory, iTunes 6.0.5. I've tried reinstalling iTunes, but it's been no help. I read on another message board about a similar problem, and that the person solved it by installing an updated version of QuickTime. I didn't really understand what QuickTime has to do with the problem, but maybe it has something to do with it. Any help would be greatly appreciated.

one more thing...

I forgot to mention something that is probably important. I just changed out my hard drive (or more accurately, my friend who knows what he's doing changed hard drives), and the problem arose immediately afterward. I'm actually not using the new hard drive yet, b/c I'm having a problem copying over my the contents of my old HD to my new one (that's the subject for another thread). I'm running off of my old HD externally.

Anyway, I realize it may be significant that the problem started just after I disassembled the computer.

How do I repair permissions?

But if it was the connection from the speakers, wouldn't iTunes play, but no sound come out? In my case iTunes simply doesn't play. And again, the volume image doesn't come up when I press the volume keys. That's what made me think it's not a hardware issue.

Thanks for posting your solution. I'll try reinstalling my OS as well.