I had this issue a couple of days ago...I ended up hitting the restore to factory settings to sort it...I have no Idea why it did it but it did and that was my fix.
This issue popped up on me out of nowhere about 3 months ago.. I reformatted the hard drive and reinstalled Windows XP with all updates(SP3, etc)..
Installed the latest drivers for the motherboard and the rest of the hardware(VideoCard, Soundcard, etc).. Same issue as before..
Everything is set properly on my end as far as my 25 years of computer experience can tell me..
The exact same settings worked before and now they don't.. Even tried rolling back to an earlier sound card driver(Audigy 2) as well as the original driver from the
cd that it came with.. No joy..