I am having the same issue here. I have Home Premium x64 on a nVidia nForce 680i mobo with Realtek HD audio. I have the R1.86 patch installed.
After I finish installing the patch, my 2.1 speakers work fine, but as soon as I restart the speakers are not detected. I have tried uninstalling, reinstalling, reinstalling with vista's compatibility mode reinstall option, but no joy. It only works the one time before I restart.
The Audio Devices panel shows the speakers are not found:
Any help with this would be much appreciated.
EDIT: After looking a little more I found this post:
http://www.vistax64.com/vista-hardware-devices/62726-realtek-high-definition-audio-problem.html This solves the problem.