the only way to get a digital out to send 6 channels of audio (and a reciever to read it) is to have the audio in Dolby Digital format. Dolby Digital is a lossy compression scheme that allows you to keep acceptable bitrates even when playing back 5.1 channels.
you can have your software DVD player pass the Dolby Digital signal (not decoded in other words) to the soundcards, which sends it out the digital port.