The problem is occuring on new 15" and 17" powerbooks. There is no known fix and Apple has not made any fixes either....or acknowledge the problem for that matter.
There was a discussion on this thread towards the bottom (post #9)
https://forums.macrumors.com/threads/167336/
I posted links to threads on apples discussion board on post #11.
There is no consensus on whether or not its hardware or software related. There is a chance that if you get an external sound card, you will still hear the problem. Your options for sound cards are a pcmcia soundcard or usb external soundcard (M-audio makes one that is mac compatible). Since your going 5.1 logitech, your best option in my opinion is M-audio sonica theater, although pricey, it will give you 7.1 analog.