I now have the same problem (it just developed over the weekend, while playing an MP3 with SoundJam and opening QT 5 at the same time.) I zapped the PRAM (Hold Option-Command-R-P during bootup) that did nothing. I trashed the QT Prefs, the Sound Prefs, still nothing. I guess eventually I will reformat, Zap the PRAM and start over. This is a horrible, insidious little bug.