Ok, I am not sure if I am just confused or just way off here, but it seems like everyone is missing what he is asking. He is trying to utilize the 3 analog 1/8 (line outs - front/rear/sub).
I am also not sure if you are aware of what these outputs consist of. Let me break it down this way. Have you ever used headphones? They are in stereo, well most of them. I think you might be confused, but correct me if I am wrong. These 3 outputs are not "MIXED" as you put it. Each one consists of a left/right.
I hope that made sense. If not, let me know. I also have the audigy that I just picked up for my new system.
I plan to try these out: http://www.monstercable.com/productPage.asp?pin=135
There are cheaper ones that you can buy, you can also go to radioshack and probably pick some up for $5. I perfer to use quality.
Again, maybe I am way off and confused. Please let me know if I am!