Software solution to use one set of speaker for multiple sound-output devices

Question

In the room I'm currently working in, I have a:

• Primary computer, running Windows 7 - connected to speakers,
• Secondary MacBook, running OS X Lion,
• "TV-computer" running Linux/XBMC or similar..

..which are all connected through my wireless/wired home-network.

Which works great, my problem, however, is that I only have one pair of speakers (5.1). I'm looking for a software that can make my primary computer act as a "sound-switch" over my network. A software that makes me able to select from which sound-output the speakers should play from, preferably from all devices.

I hope you understood what I meant. Any help is truly appreciated.

EDIT: Like Steve said, the use of a mixer would probably be preferred, but this is what I am trying to avoid.

Answer 1

After some heavy Google usage, I found what I needed: AirFoil. It's working simple and awesome, exactly what I was looking for.

Answer 2

This is not a software solution, but is in some ways simpler. I use a hardware mixer called pMix

I got mine for about $35 (the link shows $80 list price). You can use any one or all three of the inputs by adjusting the individual audio level knobs.