Many instruments have fixed notes tuned according to equal temperament, but some don't. The most common examples are the violin family, the trombone, and the human voice, but one can also find fretless guitars and bass guitars and I'm sure there are even more examples of non-fixed-note instruments.
Yesterday I had a debate with my guitar teacher about the role of muscle memory vs ear-based micro-adjustments in non-fixed-note instruments. He is inclined to think that well-trained players can hit the right note instantly, based solely on muscle memory. I am more inclined to think that ear-based micro-adjustments after the note is first hit are crucial, even if the player may be unaware that they are doing this (e.g., on a violin, the pitch can be changed just by a subtle rolling of the finger, or even by pressing slightly softer/harder to change the tension on the string).
This question could be answered scientifically by having well-trained musicians perform a melody with and without being able to hear themselves, and comparing the results. Is anyone aware of any such studies? I would also be interested in the perspective of musicians who play such instruments.