1. Save it to base/data/sound/music
2. On the start server gui, click the button called "Music List"
3. Find the song you just added, and tick the checkbox next to it
4. Start a server, once its loaded, open the brick menu and select the music brick from the special menu
5. Place it and hit it with a wrench, choose the song and hit ok.
If you aren't adding a new song, just do 4 and 5.