[cwa107]

The problem is directly related to the drivers being used under XP to support the built-in soundcard. It is not a hardware problem, otherwise you would see it also occur in OS X.

Unfortunately, what you were told by Apple support is absolutely true. Bootcamp and its corresponding drivers are still in beta, which means they are still under development. Nothing short of rewriting/debugging XP drivers for your Mac's soundcard is going to correct this problem - and unfortunately, as you might imagine, most of Apple's development team are focused on Apple software at the moment and less on development of XP software.

Eventually, Bootcamp will be released in full production and then you can expect to receive full support for bugs and problems like those you've experienced. Until then, I'm afraid you're kind of stuck. In some ways, I wish Apple hadn't done a public beta with Bootcamp since it's such a high-profile product. People have certain expectations and not having fully digested the fact that it is in beta (under development) end up being disappointed.

You actually might see better quality running XP in Parallels. Have you checked into that yet?