I had the same issue.
But mine was only during Video Conferences and only with a Creative X-Fi soundcard.
It would cause the system's master volume to drop to 25% each time any input was received on the microphone.
So for example I'd type too loud on the keyboard, the mic would pick it up, and suddenly my music had dropped to 25% volume.
I "solved" it by removing my Creative card literally throwing it against the wall and using the built in audio.
PS - Original thread
here. I might have been somewhat annoyed when I wrote that considering it was my second creative card with issues.