I waited patiently for a fix from Nvidia but i dont think it will come. The problem
After upgrading to nVidia driver 260.19.29, Everythign works great but after X closes the first time after a clean boot, audio dies. Apperntly im not the only one with this problem. So i read and i read and found nothing.
Basically
For all drivers >= V260:
* sound over HDMI is working exactly once after a reboot
* if I try to restart X, im losing sound permanently until I reboot the machine
But:
* if I stop X and exchange the driver by an old one (V195 or V256) WITHOUT rebooting, im getting back sound.
This does not work for any Driver >=260, swapping the driver back to one of these Versions still requires a reboot to get sound working again
No Solution? Only think you can do is run 256.53.
You can find it here
http://us.download.nvidia.com/XFree86/Linux-x86_64/256.53/NVIDIA-Linux-x86_64-256.53.run