Media for Computer control of an audio multiplexor

I am trying to multiplex a number of radios into the 'Line In' on my computer sound card.

I want to send commands from the PC, through the RS232 serial port, or the USB.

I want the radio signals to be switched according to the commands from my PC.

I need the switching to be 6 stereo signals to 1 stereo signal.

Has anyone done this? What is the cheapest, easiest and most flexible way to do this?

I've never used X10 stuff, and would like to avoid it, if I could easily do this with a simple USB device.