Digital out doesn't work

I have Hercules Digifire 7,1 soundcard and i have connected it to my Yamaha Rxv-630 with a optical(toslink) cable. When i look dvd movies with powerdvd everything works allright. Yamaha get the signal right and it is Dolby Digital. But when i listen mp3 or something else i don't get sound at all. If i change the settings of my soundcard i get sound correctly from mp3 and other file types, but if i now watch dvd movies i don't get the sound as dolby digital, but as a stereo. Please help??? I want to get the sound from dvd movies as dolby digital or better and mp3 and other files as stereo or whatever.