I am every day confronting this question and I have no answer that will either be convincing or satisfying.
Perhaps it is easier to declare who has "it" instead or trying to define what "it" is
Miles had it
Chet had it
Louis had it
Bix had it
African tribal music has it
Japanese folk music has it
Japanese drumming has it
Zen flute playing has it
Whale-song has it
many more than I can list here have it
It is everywhere if we look for it and yet not everyone has it, although it appears that all are capable of it if they choose to be.
Often we search for it and reach for it and it eludes us but the pull is so strong that we never give up the search for it for in the reaching for it we do sometimes attain it and when that happens we are uplifted and listeners are fulfilled and they are filled with wonder.
It is never an intellectual exercise but instead it is an emotional connection and the notes we play are the vehicle.
Musicians who can attain it look into their own souls and use their skills to represent what they find there, the listener receives it and if the musician has attained it, it reaches in, to the very heart and soul of the listener.
Then the listener begins to dance or they begin to weep or they are transported to a different place a better place, but always they are moved beyond words.
Words are sometimes used to try to describe it, words such as hypnotic transcendental meditative but they all seem to fall short.
I have been lucky enough to have witnessed it many times but I have never been able to quantify or define it.
Always it arrives when it is not looked for as though it is in hiding and must be tempted out like a frightened deer or a mouse, but seeking it out and trying to illuminate and bring it out scares it away.
It is an experience like dance and is fleeting and is gone before it can be annotated, can we annotate a dance can we annotate colour can we annotate fear can we annotate pleasure, we can perhaps annotate the actions of something, for sure we can annotate a note in a stave or a phrase but the emotional experience of it cannot I think be annotated.
So what is music, perhaps all we can really say is this - Music is an emotional experience that envelops us and moves us to a different and often a better place.