Anduril

I just got some of the promedia 2.1s, and when i hooked them up to my computer the 2 speakers work fine, but the sub doesnt do anything. the same thing happens of two different computers, so it doesnt seem to be anything wrong in that area. Any suggestiong? Also, the volume knob isnt working properly, when I turn it up, it gets quieter. It is loudest when it is set to be as quiet as it goes.