1. Redhat 8 is a bit old, your FX 5700 wasn't around when it came out. The solution is to download and install drivers from nvidia.com, and configure your system.
2. You may need to place yourself in the audio group to get sound. Check the permissions of /dev/dsp. If that's OK, see if a sound driver is loaded: lsmod. You can check which driver should be loaded by:
and look for "audio controller"s. Hopefully loading the driver is only a modprobe away.