Need help setting up home small studio

Hey im new to these forums so ermm yeah..lol
k well my bro has got 2 decks, mixer, cd deck, tape deck, and amp plugged into his mixer, he's mixer record goes into my line in my computer (for editing purposes, plus he's amp is not very good and he use's my 5.1 for he tinny sharper sound) and when i plug this into the line i can only hear the sound (from 5.1) out of my front two speakers, now this is understandable because of the line and the type of cable. only can carry 2 channels, is there a way i can make it come out of all my speakers (5.1) without having to keep changing my settings etc....so basically can i make it so tha tmy line is able to make it come outa all ma speakers? do you know of any programs that i can do this with....?

Thanx in advance

Regards Chandler