Really interesting game! As a music composer, always appreciate a game centered about music :D
In terms of audio feedback, I think leveling the instrument SFXs and BGM would've balanced it nicer. In particular, during battle, the damage SFXs tended to overpower everything else. Would've also liked the instrument sounds to differ more (ie. guitar plays chords instead of notes, pizz on strings etc. & more notes / keys / tonalities of the BGM for different levels to create some variation/interest)
I think the implementation / fades between some tracks were a bit buggy but sounds amazing otherwise. The strings in the main battle background music could be a bit tighter, it annoyed me a little they were playing a little behind the rest of the orchestra/track. Compositionally, the BGM tracks all sounded really good and high quality, was impressed!
Favourite part for me was the graphics, really appreciated the aesthetic and idea of building the instruments using parts of them.
I agree with someone else in saying that a lot of time spent on the game is doing nothing but waiting for the waves to be over, some more interactivity would've been nice and larger difficulty as I didn't get a sense of much at stake.
Overall, very innovative way to incorporate a musically and visually aesthetic design into a cool tower defence game! Congratulations on the work!