On X-Fi cards, there was something called CMSS, which when checked enabled stereo audio to upmix to fill all speakers, rather than just stereo. Kind of like what Dolby Pro Logic does I guess.
I'm not sure if there is something similar on Asus cards. I've had three Xonar's, but never any 5.1 speakers at the time, so it's something I never bothered looking into.
If there isn't then switching the output from 2.0/2.1 and 7.1 is really the best option.