I have an asus p5n-d motherboard with on board Realtek ALC883 audio. My 5.1 surround sound/dvd Receiver only had AV inputs and optical spdif in so i went out and bought a optical spdif cable and hooked it up. It sounds great and everything but its only using 2 speakers. On my reciever it says it has dolby digital and I'm using vista 64. Is there a way i can use all 5.1 channels"