If you want a music change to count towards a significant modification, make sure to create the music yourself and make it very clear that you made the music in your final submission description. Same thing for the new skins for player and enemies, make it clear whether you made the new sprites or where you got them from. If you are retheming the sprites in the game from existing sprites and not your own, be sure to swap out most of the sprites in the game. Be sure to make your new levels unique from each other.