I have an unattended installation of Windows XP that I slip-streamed Service Pack 3 into.
On an EVGA motherboard I was recently loading with Windows I could get everything working except the audio.

In the device manager I had 1 unkown pci device item listed.  Had to be the audio right?
Well I used the cd that came with the motheboard, no luck.
I went to Realtek’s support page and downloaded the latest driver, still no luck.

No matter what I did the PCI device would not update to the audio.
Finally on some sort of flash I googled “XP SP3 HD Audio“.

One of the results returned this page which talked about the exact problem and had a download to fix it.
I’m making the download available from here in case their site goes away.
I do not vouch for the file, all I can tell you is it fixed my problem.  Download from here or there at your own risk.
So, apparently the PCI device isn’t actually the audio card itself, but a UAA bus driver for high definition audio.

I found on Microsoft’s site, mention of this, and had downloads for SP1 and SP2.  Their site said it wasn’t needed for SP3.
Wrong!  Once I pointed the PCI device at the update and it installed itself, the Realtek HD Audio device was detected and it proceeded to install the drivers I had installed earlier.

Strange and irritating, but I’ve got audio now!