Mute volume puzzle

[terrypin]

I've been trying to isolate the cause of this odd problem. I'm regularly finding the Mute All option in the Playback Volume window on my XP PC enabled. I think it's probably some side effect of one of the several applications I'm using. One candidate is MM. It happened recently a few minutes after I played a few MP3s and then imported one into my video editor (another candidate). Does this ring any bells with any MM users please? Is there some 'Mute all sound' hotkey that I might be activating accidentally?

Terry, East Grinstead, UK