not entirely sure, but have you downloaded SP2? If so, how "new" is the sound card? Maybe you can search for XP compatible drivers.
or the even more strangely obvious thats over looked, is to check your sound settings uner controll panel (the speaker icon) and see what your default audio settings are, sometimes a reinstall sets those to default, and you need to select the card to use, or it even may be on mute.
im not real sure what else, thats all i know to chk. Hope it helps or leads you to the answer.