That's a cool and fun game! I made all my way until level 10 and like the idea so far, nice song, sound effects straight to the point and nice UI.
A suggestion: I feel it became harder too fast in the first levels. Maybe if you gave more time for the player to actual learn with more than one level at the beginning, it could be helpful. One more thing, as an extra bonus: you can detect player has been trying for too long and then give some hint, such as bliking or playing a sound at the right time or the like.
Best luck to you folks!