Connecting the audio output of your
Media Center PC is slightly less complicated than connecting the video, simply because there are fewer choices. In fact, you can boil the choices down to two:
If you're using your Media Center PC to watch TV or DVDs with surround sound, use an optical or coaxial digital connection (either will work fine).If you're using your Media Center PC solely to listen to stereo music, use the stereo (right/left) analog audio connection.That said, the most common type of audio connection these days is via optical digital cable. This type of connection transmits the digital audio signal across a fiber optic cable, resulting in the cleanest form of signal transfer currently available. (Just to clarify -- both optical and coaxial connections deliver the same quality of digital audio signal -- although you get less electrical interference with an optical connection.)
Optical digital connections transmit multiple channels of digital sound, which makes them ideal -- and necessary -- for transmitting 5.1-channel and higher surround soundtracks. Obviously, the digital connection can also carry a stereo audio signal or even a mono television audio signal. If in doubt, just use a digital connection, and you'll be good for anything.
Michael Miller is a writer and commentator on technology and digital lifestyle topics. 
