OP: You mentioned earlier that it plays the startup sound and then plays a few seconds in WMP (Windows Media Player) and then stops. Does WMP stop as well? Or does it run on merrily, thinking sound is still playing?
The reason I ask is that if WMP hangs, then the problem could be a DMA (Direct Memory Access) channel or IRQ (Interrupt ReQuest) conflict. The sound could be sharing DMA or IRQ with another device (ethernet or some special device you have in the machine) and one of the competing drivers is not playing nice with the sound card.
This can happen for many reasons:
1) BIOS could be setting up the DMA or IRQ channels incorrectly.
2) One of the devices may not know how to share.
3) One of the drivers may not know how to share.
4) You might be using one device a lot and it could be stealing so much bandwidth that the sound device doesn't get any. (this is unlikely)
Things to try:
1) Remove all but the essential hardware from your system: Keep video card, hard drive, DVD and unplug or remove every other device. Then boot and see if sound behaves.
2) Check the BIOS settings and disable all extraneous hardware that you don't need. The menu you want is often called, "super I/O" or "onboard devices". If you don't use a serial port, disable it. The same goes for parallel port, floppy controller, PATA (if you use SATA only), a second ethernet port (if you happen to have two and don't need the second), MIDI and Joystick. Doing this might free up resources so your audio doesn't have to share with anything else.
If option 1 works, then add things one at a time until the audio fails again. Then you have your culprit. You might be able to get it all working by following 2) above. If not you can always check for a new driver from the culprit manufacturer.
This is just a stab in the dark and really only applies if your sure everything in software land is working.
Raydude