MP3's only on one channel?! Need help!

Hello fellow DJ's!
after many months of playing with virtual DJ I decided to buy me some real equipment, so i went to a store and got me mixvibes (DVS LE s/w), 2 vynil deks ( dj-tech vynil usb 20) and a mixer (dj-tech mx 2ch DJ mixer).
that was yesterday afternoon, and i spend the following 6 hours trying to set everything up; but for some reason i just can/t get the mixer to work properly.
here's the problem:
I connected everything the wa it is supossed to be, i can play mp3's on the turntables and scratch them, and i can also play real vinyls, but for some reason the sound is only on one of the channels!
when i move the crossfader to the left, i can hear the music both vinyls are playing, when i move it to the right all i hear is silence.
when i play real vinyls the crossfader etc. is working properly, its just the thing with the mp3's
the my lcd's are set to output one and two, the sound options are set to multi channel, "device" is set to "usb audio" (this is my external soundcard, a Zalmann 5.1 usb soundcard)
on the vilyl tab i can see both decks running.
regarding the wiring i set both turntables to "line" and are connected to the line input of the mixer. the phono inputs are connected to the two soundcards.
ANY help would be highly appreciated as i really tried everything i could think of and don't have any idea how to get it working properly!
Sam
after many months of playing with virtual DJ I decided to buy me some real equipment, so i went to a store and got me mixvibes (DVS LE s/w), 2 vynil deks ( dj-tech vynil usb 20) and a mixer (dj-tech mx 2ch DJ mixer).
that was yesterday afternoon, and i spend the following 6 hours trying to set everything up; but for some reason i just can/t get the mixer to work properly.
here's the problem:
I connected everything the wa it is supossed to be, i can play mp3's on the turntables and scratch them, and i can also play real vinyls, but for some reason the sound is only on one of the channels!
when i move the crossfader to the left, i can hear the music both vinyls are playing, when i move it to the right all i hear is silence.
when i play real vinyls the crossfader etc. is working properly, its just the thing with the mp3's
the my lcd's are set to output one and two, the sound options are set to multi channel, "device" is set to "usb audio" (this is my external soundcard, a Zalmann 5.1 usb soundcard)
on the vilyl tab i can see both decks running.
regarding the wiring i set both turntables to "line" and are connected to the line input of the mixer. the phono inputs are connected to the two soundcards.
ANY help would be highly appreciated as i really tried everything i could think of and don't have any idea how to get it working properly!
Sam