The art is gorgeous and the concept is great! There are some issues that would be easy to fix and would help the game enormously.
I have to say that the standard bindings did not work for me at all. Space and WD to move felt like the control scheme of an FPS and wasn't natural or comfortable at all in such a tight and fast platformer. WASD worked a bit better, but it wasn't until I changed the controls to arrow keys that things really clicked, since it freed one hand to only focus on banjo and spring and the other to focus on movement.
The idea of using the banjo to summon platforms is solid, but it ended up being a guessing game, not only to where the notes would end up appearing, to if using the banjo would activate them at all. Missing one or the other meant I had to wait until both my banjo playing animation and the actual notes reset, since there was never time to try more than once per activation, and the banjo only added platforms at the moment of use. I think, instead of having the little creatures that you activate with music, the game would be far cleaner if the notes always appeared where they would be, but were only solid if the banjo playing was active.
I also felt an immense urge to use the spring while jumping. I realize that this would be a really brutal change to the game design and would probably create issues with the physics, but it's something to consider if you ever expand on this project.
All in all, great work and I hope you keep at it.