2 machines into 1 sound system

I am trying to connect 2 PC's into my Monsoon speaker system. I have a splitter cable but am having trouble using it. When I plug both my Soyo Dragon+ and my Asus A7N8X DLX into it the splitter, the Soyo sound continues but the Asus sound stops. If I unplug the Soyo the Asus plays fine, plug it back in and the Asus quits. I've tried reversing the connection at the splitter but there is no difference so it's not the splitter. I have also plugged the Soyo into the Asus mic jack and then I get sound from both but there is a speaker humm and a reboot of the Asus causes me to have to reconfigure the BIOS. How can I make this work?

put the line out from the soyo to the line in on the asus (not mic) and then the line out of asus to the sterio. can i ask what you/re doin with it?

I just want to have sound out of both through decent speakers. I wanted the splitter so I wouldn't have to have both on to hear the second one.

go and buy a switch box so you can swith between the two - problem solved.

http://www.maplin.co.uk/products/module.asp?CartID=030809131108382&moduleno=21301&Products=1
something like that - it allows you two switch between 3 inouts and has two outputs. It keeps the inputs electically seperate so you wont have those problems.