I've never noticed this before 10.5.7, and it's only happened to me twice, both in the last 24 hours. Since it is random, I can't replicate it, but I can't confirm if this happens when you change the system volume, which used to happen on my old 12" PB. That problem was apparently caused by high CPU load messing up the volume change. It's all in the title, the right channel mutes (both times this happened it was left biased) completely, and checking Sound Prefs. shows that my output balance slider is all the way to the left. Has anyone else had this happen to them?