I've been having a lot of authorization issues with iTunes since a recent upgrade, so when this morning, iTunes started garbling playback on ALL my music, I panicked thinking it was another bug. It sounded like the speakers were underwater.

Fortunately it turned out to be a USB issue. I use USB headphones by Plantronics, and they seemed to have lost proper connection somehow.

If you have the same problem on a Mac, and are using a USB headset or speakers, try opening the Sound control panel and switching Output away from whatever USB output device is currently selected, to Internal speakers and back again to the USB device. This fixed the problem for me.