Kind of odd how SP2 has done weird things to people's systems. I have 7
computers here that I have upgraded and not a single one had problems
that have been described. I was actually hoping some of them would so
that I could report them or solve the problems that others have been
seeing. No dice though.
To the OP, just try reinstalling the latest sound card drivers. Try
this: Uninstall the sound card drivers/software, remove the sound card
from Device Manager, reboot, then installing the latest drivers.