At this point, there are two things that I can suggest:
- Try completely uninstalling the audio driver, reboot the system, then attemp to reinstall (double-check that you do have the proper audio driver beforehand, too). Reboot the system after the installation, and try your audio again. Be sure to check the "Sounds and Audio Devices" properties under the "Audio" and "Voice" tabs to make sure you have the proper devices selected for audio output. You can find this by checking the Control Panel, or right-clicking the volume control icon in the bottom right corner of your screen.
- Failing that, especially if your install of XP is 6 months or older, you may want to look into backing up all your files and reinstalling the OS. XP starts to decline in performance around the 6 month mark and will become slower and more unstable as time goes on.