As majuh said, you need to check the settings in the Realtek driver interface aswell. Also, if 5.1 still doesn't work properly after that, I would suggest to delete the audio_options.xml in Documents\My Games\SimBin\RaceRoom Racing Experience\UserData or even delete your complete UserData folder and see if it fixes it.
I would try to uncheck the "Enable Audio Enhancements" box under the advanced tab, and try RR that way. It's mostly so you can add environmental-type effects to audio playback, and I always have it disabled unless I want to mess around. You will still get proper 5.1 without the possibility that some "enhancement" is being added that you aren't even aware of. It's worth a try.