most sound cards will replicate sound form front left and right on the back channels
this also depends on some apps, how they handle the extra channels
a dvd with 5.1 audio in vlc will help you setup the audio, in the audio menu under tools you will find how it exports audio and the mixer audio levels, atm i don't have a 5.1 audio setup so can't play with levels much
if you change in vlc form simple settings to all, in alsa you will see stereo, 4.0, 4.1, 5.0 and 5.1
other apps handle audio differently but listen to normal music, 2.0 music in 2.1 speakers is normal, even if you have more speakers unless you force the app to replicate same channels on the other speakers