Rock Your World with Firefly
No, I'm not talking browncoats and spaceships. Unfortunately, that ship has sailed. If you're the musical type, however, installing a Firefly Media Server is fairly simple. It was renamed from mt-daapd, so your distribution still might call it that. After a quick install, visit the Web configuration, usually at http://localhost:3689 with the default login mt-daapd and password mt-daapd. You can configure your music location, smart playlists and just about every other aspect of the media server. Then comes the cool part.
On any software or hardware on your network that supports daap (often known as the iTunes protocol), you should be able to play your music remotely. Firefly does a decent job of scanning your music collection and updating the clients on the network. You can add m3u playlists, and Firefly will serve up playlists as well.
I find the best way to listen to music on XBMC is via daap. It makes configuring playlists and adding media simple. It's also cross-platform, so those folks using actual iTunes can listen to their tunes as well.
UPDATE: Thanks to OctaneZ in the comments for pointing out an active fork of Firefly. Since Firefly hasn't been updated in a while, it might be worth checking out. (Of course, my music collection hasn't been updated in a while either, but that's another story altogether!)