I do have the same problem. the nvnet drivers work perfectly, and the nvaudio drivers used to work before I reinstalled linux.
In addition to the above errors, I get this in dmesg:
Nvidia + AC97 Audio, version 0.24, 01:34:26 Jul 22 2003
Nvaudio: in Funcction Nvaudio_init_module
I looked at the code that produced this output and it obviously failes when registering the pci driver ( btw: there's a typo in the error message *g* )
as for the modules.conf, I merely left it untouch, except for the changes the readme told me I should do.
I have the i810 drivers compiled in the kernel, and sound works already, but I *need* the spdif support. I know I had it working with nvaudio, and I dont think I did anything special to set it up back then.... any ideas?