Very cool and unique concept! I'm really liking the concept of this :) The slight fisheye and grain effect is also very cool hahaha
I'm not really a musically inclined person so I only played the first few easy levels. Here's some of my observations from the early levels I played:
-Maybe when the you nail down the correct rhythm, there'll be a short music that fits the rhythm played. This incentivizes the player to complete the level just to listen to the music when they completes the level!
-There's no indicator for when you failed the puzzle. Instead of just stopping there you could maybe add a distorted sound to indicate its the wrong rhythm, and some animation too for the box.
-The graphics could be more interesting, maybe using the musical staff as the platform or something, or even create your own little musical world to decorate up your game!
-Adding a reset notes button would be very helpful, especially in the late level where you have alot of notes to add.
Overall, really great idea :) Good work on the game!