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 hypothesis is that music is a vestigial trait, and that music originally evolved as an extension of spoken word-based language, providing the ability to express meanings which, at that time, spoken language could not express.
I have also developed a hypothesis about the fundamental meaning of music.
The Fundamental Meaning of Music
The fundamental meaning of music is that music motivates a listener to think about a particular situation as if certain things do not exist, or, if they do exist, as if they do not matter.
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 a component of language
Music evolved as an innate extension of arbitrary (non-innate) spoken word-based language. Music provided the ability to express certain meanings that spoken language could not provide.
Music expressed meanings "beyond the here and now".
Spoken word-based language had difficulty evolving to express meanings about things beyond the "here and now", because spoken language requires language learners to acquire the relationships between words and their meanings, and such meanings are naturally difficult to acquire, given that the speaker of such meanings is speaking about something that is not there, so the language learning listener has difficulty guessing what the speaker might be speaking about.
Music evolved as an innate extension to spoken language, to express a set of meanings relating to the "beyond here and now". The innateness of musical meaning avoided the difficulty of acquisition.
Music expressed meanings which it still expresses today
When music functioned as part of language, it expressed the same meanings that it expresses now. The difference now is that music is not communicative, and it only express meanings in relation to thoughts that are hypothetical.
Music is not recursive
It is likely that music expresses multiple dimensions of meaning. However, unlike word-based language, music does not construct more complex meanings from the syntactical composition of units of simpler meaning.
Word-based language eventually expanded to subsume the function of music
Music was able to innately express meanings that word-based language could not express.
Once such meanings were being expressed, it became advantageous for word-based language to incorporate those meanings, and selective pressure existed for our human ancestors to evolve the ability to acquire such meanings without the assistance of music (although music may still play a role in the acquisition of those meanings in spoken language?).
Once word-based language included such meanings, music was a less efficient and flexible way to express them, and music became redundant as a component of human communication.
However, music still serves some secondary functions, and thus continues to exist, but in a reduced form (like ostrich wings).