I've noticed if you turn the volume to the highest setting on the 12" MacBook, then test the OS X "error" sound effect, it comes out distorted. It starts sounding distorted after about 2/3 to 3/4 to highest volume settings. Once it's at highest, it no longer even sounds like it. The speakers have a slight "pop" sound to them when playing the sound effect is distorted. Is this normal on other people's machines? You can test this sound effect by going to System Preferences > Sound > Sound Effects > Funk or just causing it to play somewhere within OS X using the keyboard and entering a bad input.