Did you install drivers for the sound card?
Were there any issues before your reinstall?
okay, next up. Try external speakers and make sure that it is not simple failing speakers.
Also update your sound drivers if at all possible. It may be a bug.
Get a cheap USB sound card and try to use that for a while. It will bypass the SPU on the motherboard and rule that in or out as the cause. I am betting it is the cause at this point though,
I have had a similar problem recently, and I just took it into Best Buy after many grieving hours of trying to figure out what was wrong with it. I'm not entirely sure what they did, but it seemed to do the trick. Hasn't bothered me since.