This is a quick mod I threw together in 2 days for fun. It plays .mp3 files and loads playlists from folders and .m3u files.
Controls (All Rebindable):
Home: Open menu
Page Up: Music Volume Up (Ctrl-click acts as Next Song)
Page Down: Music Volume Down (Ctrl-click acts as Previous Song)
You can create a playlist by selecting a name and pressing the button in the corner. Selecting a .m3u file crawls it and picks up all .mp3 files it can find. Selecting a folder has it look through it and sub folders for all .mp3 files. Streaming and extended m3u is not suported yet.
You can then select a playlist and double click a song to play it. You can move or disable the HUD and change the play mode (Track, Repeat [Track], Loop [Playlist] and Shuffle) in the options at the top.
Holding Shift + Ctrl enables the developer settings if something breaks, try resetting the thread.
NOTE: The playlist selector appears in a separate window. If your game freezes when you click the "Select Playlist" button, try alt tabbing and seeing if the window is there.
If you have any problems with the mod or just want to contact me, you can find me on Twitter (@Vazkii) or IRC (#vazkii on irc.esper.net).