I have a Macbook and am running 5.1 surround out of it.
For the life of me i cant find the exact cable i am using but i hope this points you in the right direction.
Check this
link out.
I have the same or very similar cable running to a black splitter box.
The 2 red and white Belkin cable in one side then the brown, black and green cords from my 5.1 system in the other.
Its been a while since i have purchased this 5.1 but im pretty sure the splitter box was with that and not the Belkin cable.
Here is image to help out.
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Im sure there will be other brands to look for too.
Hope this gives you a course to take.
Excellent sound once you have it hooked up.
Im running my Eye TV though my Macbook and as well as watching DVD and i always think im in a theatre
You dont need a external sound card