optical audio problems

I have just recently bought a set of Logitech z-5500 5.1 speakers for my PC. I am using the Abit Fatality An8 sli mobo, which, if you do your research, has optical output. I am using windows x64 pro edition (dont know if it makes a difference) with all the latest audio drivers installed, and I use Intervideo WinDVD 7 Platinum to play my Dvd's. Anyway, to the point - put a DVD movie in my drive, go to play it with optical audio output - and surprise surprise, no sound. Change all the settings around, try again - no sound. But as soon as i switch to 6 channel direct input, all is fine. Why doesnt my optical connection work???? I really want Dolby DTS sound