Thanks for liking the game! The laser is also my favorite one :)
The game has no ending and neither will you die. We want players to explore various combinations of the music and when you find the one you like, you can just put it as the background music and go afk.
The energy level is a condition that many music samples depend on. Some only play when the energy level is above/below certain value.
As for chanllenge, maybe it is a good idea to have a special mode where players bring the music designed in the free mode to a test. The music cannot be further modified and the player needs to survive as much wave as possible.