Well, if the source material (mp3's?) is only stereo, you will have only stereo output (front left and right). What you need is a sound card or DAC that can upmix stereo into 5.1. Home Theaters usually have Dolby ProLogic II that does the upmixing. Some Creative products have CMSS (Creative Multi Speaker Surround). I don't know about other 5.1 sound cards.
What do you have?
BTW, Winamp can support AC3 files with the corresponding plugin, which is true 5.1.
-psyrex