No sound from AC3 sound source

I have an Audigy 2 which souldn support AC3 decoding, shouldn’t it? Nevertheless my speakers (simply stereo ones) don’t put out any sound when playing AC3 sound from Windows Media Player or PowerDVD.
Could someone clear me up here?

Are you using the digital output on your sound card?
//Danne

I’m using the standard 2 channel analog output.
Does the Dolby Digital Decoder only decode the digital output?