According to the your thread title, you are hearing System Sounds, is that correct? Or are you only hearing sound output via the internal speaker on the Motherboard when there is some error or Bios startup?
Where are you getting the drivers from that you are using, Windows Updates, or more sensibly, the manufacturer?
Download Belarc Advisor (free) and see if its description of your Sound Card is identical to the make, serial, version, of the drivers you are installing.
Try eliminating third party interference.
310353 - How to Perform a Clean Boot in Windows XP
316434 - HOW TO: Perform Advanced Clean-Boot Troubleshooting in Windows XP
310560 - How to Troubleshoot By Using the Msconfig Utility in Windows XP
Do you want to try a parallel install of Windows, just to see if sound works in a clean install?
Don't install any unnecessary programs just some sound drivers, keep the parallel installation as simple as possible.
See the paragraph in this article.
"To install Windows XP to a new folder (to perform a parallel installation):"
How to install or upgrade to Windows XP
If it all gets too much there is always repairing windows by running it over itself. You will lose all your windows updates but your files and programs will be untouched.
How to Perform an In-Place Upgrade (Reinstallation) of Windows XP (Q315341)