A) I assume you have the amplifier grounded also.AMP is hooked up to battery and ACC. I checked that so many times
so I think AMP is connected fine.
B) Do you know if the amp is turning on? Is there any kind of an LED that would indicate it is on?
C) Are the output levels so drastically low on your sound card and the gain so low on your amp that you just can't hear any sound (essentially like having the volume all the way off).
D) What is the amp model number so we can see if we can discover any other problems.
E) Such as, if it is a 4 channel amp, it might be possible to set it in a way that it only needs input from 2 of the channels and you have the cables connected to the wrong inputs.