The big question is: "What is Music?".
It seems an easy question to answer, because we are all so familiar with music. But being familiar with something is not the same thing as knowing what it is.
When I ask myself this question about music, I ask it as a scientific question. So I want an answer which relates music to our scientific understanding of the world. And I want an answer that makes predictions about music (and for bonus points, it should make predictions about other things besides music).
My most recent (and possibly final) hypothesis about music is that music is a vestigial trait.
Music is a Vestigial Trait
Music is vestigial: it used to have a biological function, and it no longer has that biological function.
Music was previously a form of language
Prior to modern word-based languages, our human ancestors communicated in a language where music had meaning.
The unit of meaning in music-based language was the melody
A strong melody has a strong perceived identity. Melodies functioned as symbols, to which culturally-assigned meanings were attached.
Music-based language was non-compositional
Music-based language did not allow any form of composition of units of meaning to create more complex meanings. It followed that every melody, ie every unit of meaning, had to represent a stand-alone self-contained assertion (about something).
Music-based language was replaced by superior word-based language
Word-based language replaced music-based language, because it is vastly superior, in two respects:
Music-based language strongly constrained the evolution of any replacement
Music controlled the process of language acquisition, sufficiently strongly that any new system of language could only evolve as a language embedded within the music.
Also, any such replacement had to encode information entirely in acoustic characteristics of sound not relevant to the identity of music items.
Word-based language was able to evolve within these two constraints: it initially evolved as de facto song lyrics, and, words encode information in choice of consonant and vowel sounds.
(Vocal music performance depends on consonant sounds to define rhythm and vowel sounds to carry the melody. But, as it happens, the choices of consonant sounds and vowel sounds are not musically relevant, and in particular they do not affect the perceived musical quality or identity of a melody.)
Modern music is what remains of the previous system of music-based language
Music has "unevolved" into a form in which it no longer functions as a language.
But we know, of course, that music has not fully disappeared.
The disabling of music as language has occurred mostly by disabling one critical component of music-as-language – what was disabled was the ability and the tendency to assign meanings to musical items.
Music continues to exist, partly because the transition to vestigiality happened fairly recently (probably ~70,000-50,000 years ago), and partly because it continues to serve secondary functions.
For a fuller description of this hypothesis, please read Music is a Vestigial Trait.