Yeah, I've checked all volume controls and stuff, nothing is muted and they're all on normal levels. This happened about a month ago and a system restore fixed it, but all previous system restore checkpoints no longer work, so I can't use that. When I turn up the volume to full, there's a low crackling sound from the speakers, just as it happened last month, so I'm pretty certain it's not the speakers. I'm on Windows XP, BTW, with SBLive! (WDM) soundcard. I'm about to reinstall the drivers, but wanted to see if anbody has any other advice.
Try doing a system restore, you can do that by going to the help menu, and then type on the bar "system restore" and then restore to an earlier time, the safest way to do this is restore it to the last time you had sound on your computer. This has happened to me and thats how i fix it. hope that will help.