I can't find the article to reference so take this with a grain of salt: in ancient times it is believed that song was a form of letting people know you were there, as in mothers cooing to their babies while grazing. That's a really pleasant idea to me, but I believe it's far more complex than that and music/sound can't be simplified down to a single evolutionary cause. As someone who loves sound is all it's glorious forms it's distasteful to me. Why would I be content to play the piano to only myself all day if it were something as base as for getting a better mate? It goes way deeper than that.