I believe the current theory is that musical talent was a sexual selection criteria that 'blew up'. Good rhythm, a good singing voice, and an ability to remember complex rhythm were originally linked to timing and muscle coordination, and so helped to signal for hunting fitness; and to intelligence, and so helped to signal for the ability to navigate the pack's social landscape. But once sexual selection for a trait begins, that trait can take on a life of its own, leading to things like peacocks' tails and lyre bird's mating calls.