Are you trying to get 5.1 sound in games or movies? If it is movies then you should be getting 5.1 sound provided your DVD software is a full version and supports 5.1 sound (OEM versions require a plug-in).
If you are trying to get 5.1 in games then you are out of luck. The Audigy cards will only pass 2ch stereo through the digital output. The analog outputs will work fine though.
See the note above:
*Note: 99% of sound cards will not output a 5.1 channel Dolby Digital signal when playing a game. The only sound cards that will are the BlueGears X-Mystique 7.1 and some older Nforce 2 motherboards with Soundstorm onboard sound. Those two audio devices have one thing in common, Dolby Digital Live (DDL). DDL encodes all of the sounds from a PC into a Dolby Digital stream. Most sound cards, including the popular Creative Audigy and even the brand new X-Fi series will not do that. They will, however, pass through 5.1 sound from DVD’s. The bottom line, if you want to play games and transmit sound over the digital connection you need one of the sound cards just mentioned or you will only get two channel sound.