It says that it's based on omxplayer, which I believe is commonly used for playing MP3s as well as videos on the Pi. Generally any package that supports video playback supports audio as well because there's little extra effort in adding support for that. Still I said "_might_ be useful" because I hadn't looked too closely at it. If the client-server system matches exactly what the OP is after (I've got no idea if it does), maybe it would be a good starting point if not an immediate solution.
I'm thinking that USB sound adapters shouldn't be hard to get working if not recognised by default, but I might be wrong.