6.1 Surround Sound on G5 iMac

I am a bit confused with the setup of the surround sound I just bought for my Mac.

I understood that the G5 iMac has a 5.1 compatible sound card built in which uses the headphone jack as the input. I shopped around and bought a Creative Inspire 6700 6.1 system which has a 5.1 upmix switch.

When I received it I was expecting 1 input cable but there are 3 (front/rear & sub) cables. My question is, is there a cable / adapter you can buy which converts the 3 into 1?

The alternative I've seen is the external USB soundcards but that kind of defeats the purpose of the built-in card which I was presuming I could use.

Any advice and recommendations would be magic.

Cheers!