The solution to this one
I'd like to thank Vinod Gopalakrishnakurup at nVidia, who responded superhumanly fast to my bug report on this.
It turns out that under Sound Cards in the Linux kernel menu, "Intel ICH (i8xx), SiS 7012, NVidia nForce Audio or AMD 768/811x" has to be selected and installed as a module. Failing to select it at all gives the problem I reported; installing it in the kernel creates an insmod and interrupt error problem, since the nVidia module doesn't check to see if the kernel already supports the features it needs - the standard thing is for modules to look at the kernel first.
I had expected the distributed nVidia module to be self-sufficient - didn't realize it depended on another kernel module already being there. Hopefully this will be noted in the install instructions in the future?