oh for the audio you would also need an RCA stereo to mini-jack adapter
so one end would have two female RCA jacks and the other end has a headphone plug, one end plugs into audio outputs form xbox360, other end plugs into your line in on the sound card, of course your computer would have to be on then when you are playing your 360 for you to hear any sound
now if you have a better set of speakers that have more than one input, you can hookup the 360 audio right into the speakers instead and cut out the computer completely
you may even want to look into getting a cheaper receiver for your computer with a set of decent speakers, then you can hook up all of the sound outputs directly to the receiver