Most sound cards do not currently offer a digital out. A few higher end cards offer digital out. The 1/8" miniplug mentioned is an analog output, that is why a simple cable to convert to RCA connectors works. Sean's statement that the D/As on most sound cards are cheap is correct (including the SB AWE64 Gold), but many people can't hear the difference without a direct comparison (music quality is something that very few really appreciate!) (not trying to sound snobby, just stating the facts).

256K MP3s are the future and offer very good quality, assuming the food chain is good.