I have the same problem. My motherboard is an ASUS K8V-VM.
All seems to be OK but Windows 7 says I don't have audio hardware.
All I found is installing the realtek driver
R2.23 (very boring installation because I had to say exactly 25 times that I really want to install it : this driver is not recognized by Windows 7)
The sound works well until I reboot the PC, and I've to repeat this operation each time I use Windows 7.
Disactivating the driver's "signature" (in french's version) when pressing F8 during the boot changes nothing.
By the way that's better than nothing for testing applies that use sound.
I'll tell you if I find the real solution.