I had a very similar problem when I had build 6801. What i did to fix it was i went to the Realtek website and downloaded the most up-to-date sound driver for my device. From then on i had no problems with crackling or the sound not working at all.
I've always had bad luck with onboard sound and trying to listen to multiple audio sources at once, (ie: Winamp and a Game) But as iroken22 said, getting the latest driver often times will fix the problem.
Very well put. I spent a whole day just looking through and testing the computer to determine the issue. I finallly figured it out 17 hours later when I read a post online that someone had a similar problem in VISTA and that fixed it. So I gave it a whack and it worked.