Fixes based on watching a 100% playthrough

For the first time, I watched someone else beat the whole demo with all stars (that is 100% solution).

Very kind of him: https://www.twitch.tv/IcelandicBanks

There were many small things to learn on how to get the levels to make more sense.

But he also said that the music was too loud, and he had to turn it off. So I added a slider today, for the middle ground.