The music in this game really compliments the graphics, they work so well together.
And the way that most of new mechanics are introduced is also great. Gives an easy level with the new mechanic then starts increasing in difficulty. A lot jam games suffer in this catagory so good job on this part.
You might notice I used most there, that is becuase I feel this wasn't the case with the boss. While the increase in difficulty was on point, I didn't know what I was supposed to do at the start. I collected the square things but nothing happened then suddenly I hear the getting hit sound; I thought I got hit, but my health was full. then after a couple of times I realised it was the boss getting hit. It could be me being stupid but you can't blame me for being confused, it has the same sound and screen shake.
This is a small thing but being forced to use fullscreen is kinda odd
TL;DR: The game is perfect, from gameplay to music to graphics, but I think you should make the boss get hit straight away and use a different sound and visual queue.