If you have 5.1 speakers, you may be using your Line In port as an output for speakers. Many modern motherboard soundchips that support 5.1 use the Line In and Microphone jacks as rear left/right and subwoofer/center outputs when in surround mode. You may have to switch back to 2 channel stereo in order to get your Line In port open, then use the Stereo RCA to 1/8" minijack Y adapter to plug the output of the 360 into the Line In port (I think it's blue, but it might be green, look for Line In or a picture of sound going IN to the PC).