HD Audio Troubles

Hello,

I replaced two motherboards yesterday. Everything is working fine, except the audio.
I install the drivers, and my sound icon is visible, but there is no sound. I tried the kb888111 hot fix, but that just helps you detect the audio device in windows' device manager.

Here is what is NOT the problem:
* It is the right drivers
* The sound ports are not blown
* The sound isn't muted, speakers off, broken etc.
* The windows audio service is running
* The onboard sound is not disabled in CMOS setup

I am running XP Pro SP2 on both machines. I tried various online forums, IT support etc and have found no solution.
Realtek HD Audio on both boards. One is a Biostar motherboard, and the other Gigabyte P35 DS3L. I am really absorbed in this problem, and do not understand why there is no sound. I have plenty of driver experience etc, but i cannot think of a reason why it isn't working.

I would really appreciate your help as i am running against a deadline to get these machines up and running.