EP45-UD3R Rev. 1.1 sound problem

[nagonsa]

Hello to everyone, I will try to explain my problem to see if someone can help me. I have connected a home theater sound system to my pc by using optical cable, I have also gone into realtek and activate optical output so I can use it. Well my problem is that I can not put DTS output or even I can not configure sound. when I do a sound test I can only hear sound from the left and from the right and not from the back speakers or from central one. Can someone please tell how can I use that optical output or how should I connect my pc to get 5.1 chanel sound? Is it better to use SPDF or optical? Maybe the main board is wrong or something? Or the optical output is not for that use?
Thanks very much to everyone.