Hi Chris!
My background music are not custom-made, but I did pick songs that I think are suitable for my game. For example, I used joyful songs for main menu and level 1 and excited or sci-fi themed music for next two levels.
I changed the total view of level 2 by using robot building prefabs and I add one more obstacle, push trap.
I applied environment prefabs most. My level 1 is in a space station, the one showed in tutorial videos, and I added a hallway with window and changed some walls. My level 2 and level 3 are robot building and cave scene respectively.