In my case, I did have the opportunity and pleasure to directly engage with musicians, scientists and philosophers about this question. Still, whether via words, measurements, or demonstrations, the mental formation of a clear and definite answer seemed to stay outside of my reach. At a certain point, I felt forced to try to find an answer in my own way.
The first idea was then to not formulate this answer in writing. Given the indirectness of words, that would be an ultimately futile endeavour. Instead, the answer was to be formulated directly, via perceptual stimuli and induced subjective experiences.
These stimuli were then to be designed carefully: First, so as to induce aspects of music and non-music that are experientially similar. Second, to also induce transitions inbetween these aspects. The idea here was that doing this should make it possible to directly experience the differences that delineate what music is.
Finally, the stimuli also were to repeat, exactly. This would enable the listener, over time, to consciously compare the differences being experienced, and to reflect on what in them appears non-incidental.
The end result of this approach has been an Extreme focus on the limits of music.