Help, mixtrack pro works with program, but no sound! =(

Hi guys,

Recently I just got the Numark Mixtrack Pro bundled with Traktor Le 2. I've installed everything correctly, and the first few times I tried it, the sound worked and everything was fine. However, the next time I plugged in my controller and ran Traktor Le 2, the music played, but no sound came out through my computer speakers. (Note: My headphones were broken, so I only used my computer, the controller and the computer's speakers.)

I've also tried it with Virtual Dj Pro 7, and the same thing happened, the controller worked fine with the program, the music ran but no sound came out of the speakers. Does anyone know what I should do? Should I have plugged the computer speakers directly into the controller? Thanks =)