I doubt the graphics card is causing the no sound error.... Make sure that you use NVIDIA High Def Sound like nostall said; also, have you went to Sound > Properties and viewed disabled devices or whatever it is called?
Im using the realtek onboard sound card on my motherboard which is selected in sound and on device manager, I think it must be something to do with the graphics card only because ive tried so many different things, I cannot think how it can be making the sound not work its bizzare
If I put my older graphics card back in the sound comes back on, or if I boot into windows 7 32 bit the sound works fine, or if I boot win7 64bit the same its fine. (ive got a multi boot system, they wont be causing any problems as their all completely separate)
Plus if I take the audio jack out, the media player stops.