So, I think the real answer is we don't know. There going to be a lot of guesses based on SOME evidence, but there is never going to be anything conclusive because we can't really go back and time and observe the moment when humans first started to make and enjoy music.

Obviously, music can be enjoyed by more than just humans, so maybe the question shouldn't be "why humans" but more like "where did the love of music first start?" and maybe we have a common ancestor with other animals that explains that love.