hmm... with all this new info my best bet would be a virus, software issue, or driver issue. its not ur motherboard cause it wouldnt be muting itself, it just plain wouldnt work
. i also really doubt its a virus, but hey, it doenst hurt to check
here are some solutions...
1. do a virus scan
2. do a spyware scan
3. download and run ccleaner ( cleans up garbage, u might also want to run the registry cleaner also) u can download ccleaner from here
http://www.ccleaner.com/4. download and run hijack this and post a log here (download hijack this from here
http://www.majorgeeks.com/download3155.html5. if u still have ur cd that came w/ ur motherboard, reinstall the onboard sound card driver
6. if not, go to ur motherboard manufacturers website and download their latest onboard sound card driver.
this is listed in priority. ( start from the top and work ur way down, testing each step to see if it fixed the problem)