Hi again 🙂
I had a look at the Apple Support boards and this is a known issue in Mountain Lion with an Apple Bug Tracking ID. People have been complaining since at least last August. --And it sounds like it happens after the Mac wakes from sleep, --yours too? Some people have suggested doing a PRAM reset but that's only temporary for maybe a week. Others have suggested tweaking coreaudiod files --but when that works it's only temporary too, --and has unleashed (bizarrely) in some cases, a backlogged flood of loud Mail pings.
So sorry, no help yet. 🙁