For it you first need and USB sound adapter plugged into your eTRAYz USB port. More info about on which one I used here:
Its an application that builds database of your songs (can play mp3, ogg and flac files) and stream them to sound card. You can control it with integrated console based client called ncmpc. Type in console to run it after you started mpd process:
Alternative to that is a realy nice Web client called client175:
Another alternative is a Firefox addon called Minion:
Realy cool stuff. And tons of option I haven't explored yet.
Drivers for your USB card should be autostarted when your first start mpd process.