Awesome game. I had a lot of fun playing. It really shows that you don't need beautiful assets to make a game with cool aesthetics.
The idea with each enemy having their own music is great. Though I felt that the sounds enemies make have different BPM than the music making them not really fit together (I could be wrong though).
Two small issues:
Volume slider is back to initial position after reentering settings menu.
I found the sound when clicking a button in menus quite unpleasant. I think lowering it's pitch would be nice.
These are very minor though. Amazing job overall!