I don't have an nforce board myself but for sound you can use the default i810 drivers included with rh 7.3. Months ago the nvidia pci ids were added to the i810 drivers so the drivers work on nvidia stuff too. RH sets up the drivers itself (I know others without problems). Try to use sndconfig (if installed on rh).
I don't think the problem you have is related to the nvidia audio driver itself. I think you don't have permissions to access /dev/mixer. Try a mixer tool as a different user (root) and see if it works for that user. If it works for root you need to change the permissions on /dev/mixer. (I think rh has a tool for it ..., likely there's a group mixer to which you don't belong)