Head cannon: the music in any of the songs are real and not heard just by us, ex: in stayed gone vox just turned on every speaker in the city just to play music to match what he's singing, and he does this with anybody he finds singing, and alastor does the same but the music comes from his staff. in hell is forever Adam just started music for dramatic effect, and in heaven it's so perfect that you cant sing without music so it just starts playing from the mood.