You will want to use 'Audacity', a free program, to make music Add-ons.
You need to download an .mp3 file of the song you want, then open it in Audacity.
Then convert it from Stereo to Mono (there should be an option to do this somewhere).
Then cut the file down to the part you want to play (pretty straightforward when you get into it).
Then save it as a .ogg file.
Just move that file into your Add-ons, toggle it on when you start a game, and you're done.
Sorry for not being too in-depth, as I don't have the program in front of me, but Audacity is pretty straightforward, and if you get stuck just ask for help again.