I had exactly the same problem. I had the P5W DH Deluxe mobo and had several problems with it, one of them being never being able to get the onboard Realtek audio working, even with the latest Realtek drivers for the mobo from Asus website.
I was on the 64 bit version of Vista Ultimate.
Never got it to work, tried just about everything.
Eventually I eneded up returning the mobo and got another one instead - but that was not only because of the audio problem.
I really hope you figure it out and if you do please post the solution since I am curious, and hopefully that will then help others in the same situation!