i am using powerdvd 4.0, but i don't know which version do you have. if you have the same version as i do, you can go to configurations by right click on the picture. then go to audio, select dolby 5.1. then you are all set to go!
very simple, your music was not encoded in surrounding sound.
to test if the sound card and the speakers working properly, you can play a dvd movie which has dolby 5.1 lable on the box. my theory is, if it works on the system testing, then it should works fine.