You should create a variable and assign it a value based upon the number of the music you want to play. Here is an example program. You would need to change the filename at the LoadMusic function to the name of your file and it would need to be located in the same folder as the program's .exe file.
sync on : sync rate 60
global GameMusic
GameMusic = 1
LoadMusic()
loop music GameMusic
repeat
sync
until spacekey() = 1
delete music GameMusic
end
function LoadMusic()
load music "MyMusic.mid",GameMusic
endfunction
So many games to code.....so little time.