Or try another pair of speakers or headphones on the computer that's having a problem.
Try playing a different file type, like a MIDI file, and see if you get sound. I know you said you checked for muting, but double check and see that all volumes sliders are set high enough (Playback (Main), wave, midi, etc.) and that none of the check boxes below those are checked as muted (or makes sure they're checked as "Selected", depending on your sound card model).
I doubt it will help, but you can try uninstalling and reinstalling the driver. There's no harm in trying it though. What model of sound card is this? How old is it?
It might have just died. It's also possible that there is a problem with the jack on the back. In that case, you'd be better off getting a new one.
