Needs to be in your sound folder (so it's in garrysmod/garrysmod/sound, on the same level of your folders as addons and materials. (maybe it's sounds, not sure)

Ohh, so the first part is the file, the second part is the song. So it would be like:
SONGS/track1.mp3

If you put track1.mp3 inside of garrysmod/garrysmod/sound then when you put in
track1.mp3
as the parameter to the sound emitter it will run that song

if you put it in garrysmod/garrysmod/sound/hello instead then you'd put
hello/track1.mp3
in the sound emitter.