Looks like Apple’s having a little problem with most of it’s new hi-res Powerbooks. Seems the audio buffer is creating an echo loop in which the music stream is destablilized and repeats until the buffer catches up. Kind of like a cd skipping.
powerbookdefect.info: “Intermittent audio echo/loop emitted from the internal speakers and/or the line/headphone out jack. Echo/loop sessions last approximately 5 seconds, after which audio performance returns to normal, until the next echo/loop.”
For more information or if you’ve noticed this problem on your Powerbook be sure to check out powerbookdefect.info.