Check the device manager (control panel/system) and see if the audio drivers are installed or if there is a big yellow exclamation point next to the audio devices.
If the drivers aren't installed correctly, you can install them manually.
When I installed XP on my Mini, the installer quit and restarted about 1/2 way through. I manually added all the drivers to the devices that weren't working and then re-ran the installer (the BootCamp control panel installs near the end of the installer, and that hadn't installed yet).
Everything went smoothly with the installer after running it a second time.
By the way, make sure your using an XP install disk with SP2 or later already built in.
-Benjamin