Computer software/audio ?

Anyone know why a USB port would disable the audio inputs on a sound card.

I plugged the USB cable into my Outlaw 990 to recieve a signal from my computer.My computer runs TrueRta and the mic plugs into the line in.For some reason plugging in the USB cable disables the line in and discontinues the loop.

Help.Analog works but analog bypass's the units bass management and I'm trying to smooth out the crossover bump.Grrrrrrrr.