No one's asked this yet....Is it plugged in? =) Have you check for faulty audio cables or a bad amplifier? Use a set of headphones, check them on a separate CD or MP3 player first to make sure they work, then try the PC. Check your audio settings one more time. Vista should handle the audio settings itself, but XP has a much more complex audio mixer panel, it's possible software muted the Stereo Mixer, which may not show up on your mixer panel if you don't check the box in properties to make it show everything.
With the obvious out of the way, do you have a front panel audio jack on the PC? If you use headphones sometimes inserting the a jack in the front will disable the back panel jacks, try the front panel, atlernatly the front panel plug detect may have gotten stuck on somehow, you can try disconnecting it internally if it exists.
You'd be more likley to break something else more important than successfully repair the mothboard even if you could find the problem and had all the rework tools to replace it regardless of your experience.
Even if you don't have an internal slot available for a replacement sound card there are USB sound cards out there as well.