Mine does too, and I know why. My computer is well overdue for a re-install of Windows. I have that Serene Aquarium screensaver which has the bubble sound. Some weeks ago the screensaver would disappear when I moved the mouse, but the sound kept going. It doesn't really bother me, and one of those days (before 2009) I'll do a thorough cleanup.
This probably has nothing to do with the sound on your computer, but I just thought you might like to know the plight of a fellow-bubbler.