Hi!
About music issue
Do this:
1. Just add track in root scene
2. Get off checkboxes "looping" and "autoplay in music Inspector
3. Play music once. for example in root scene:
func _ready():
$music.play()
4. Connect music's signal "finished()' to the script
5. Write there:
func _on_music_finished() -> void:
$music.play()
thats all — infinity loop in HTML 5 =)