yeah, i have somewhat of the same problem with carbon in vista ultimate. the sound is really choppy during play as well as the graphics. i believe my issue is driver related (video mainly, but possibly audio as well). i guess i would say make sure all your drivers are completely up to date. www.driveragent.com
will tell you what drivers have been updated. it won't let you d/l them without a password, but it'll give you an idea of what to work for.
oh yeah, realtek doesn't currently have audio drivers for vista, that could definently be your problem right there.