Well, RedHat does not seem to load the sound module at boot time unless it needs a permanent DMA buffer, by my reading of /etc/rc.sysinit on my system. In short, unless you set this permanent DMA option in /etc/modules.conf (and it does NOT seem to be the default), you may need to load the sound module manually using modprobe ("modprobe nvaudio").
Before testing the sound, you may also consider checking to make sure that the module is loaded using "/sbin/lsmod". If you don't see nvaudio in the resulting list, the module is not loaded. If the module is not loaded, it won't be possible to open device files like /dev/mixer for it.
You might also consider checking if esound is running using "ps -ef | grep esd". I've not used sndconfig, but esd may have some of the sound device files open when you run sndconfig. I doubt that esd is locking /dev/mixer, but this might also be worth checking.
The only way sndconfig would be locked out would be if (1) sndconfig did not try to access the audio hardware through esd if it could not do so itself (likely), esd had /dev/mixer open (I doubt this), and the nvaudio module does not support more than one open (likely). Still, it is one more variable you could check.