It's not really possible, but you can give this a try and see what happens: in your garrysmod/sound folder, create a folder called "unioncity3", inside that folder create another folder called "music", and name your music file clubtrack1 which has been saved as .wav file, then select Track 1 in the map and hit play. If that sounds too complicated, it probably isn't worth the effort and most likely will cause ear-piercing corrupted sounds, as a warning.