Help! Vista Audio Volume turning down by itself?

i am using vista ultimate, running with latest updates...

suddenly, the audio volume level will automatically turn itself down, often
gradually from 100% to 80% then eventually to 0%. it does this rather
randomly, and is not linked to any software i used. once it is down to 0%, i
cannot turn it back to 100% as the volume level slider will immediately drop
back down to 0% by itself.

i have checked my system for virus or spyware and it came clean. i have also
tried to uncheck the option in the sound properties to enable other programs
to take control of the system sound... still this problem occurs..

i have searched everything and cannot seem to find what the problem this.
this is most frustrating... please help!!!!!