I'm using really basic USB speakers (ThinkVision Soundbar) and they have been working fine with my Macbook for months (Mac OS X 10.4.11). They turn on automatically when plugged into my USB port. Today, they turned on like normal, but when I do anything requiring sound output (ex., turning up the volume), they shut off. I can unplug and replug them and the same thing happens: I see the light go on, I see them listed under System Profiler and Sound in Preferences, but any sound needed turns them off (and the computer ceases to recognize them). What on earth? Any ideas?