Download this zip:

 XMusicPlayer.zip

Extract into your rtb as you would with any mod.

Open the game and run this command in the console:

exec("rtb/client/x/xmusic.cs");

A message should appear informing you that instalation has completed.

Close RTB and copy any music you wish to play into 'rtb/music' > Music files can only be .WAV

Other file types can not be added, as far as I can see this would require engine modification.

To play the music, you must have the game sound enabled in the options (Ctrl O > Audio)

Use F6 when spawned to open the gui or:

Enter the following in the console:

toggleXMusicGui(1);