after all, that the "battle" is over, why is the song of the family so fierce and loud in kino's ears? What or who is the "enemy" hi is seemingly preparing to fight?

It isn't what Kino is preparing to fight that causes the Song of Family to ring in his ears; it is to preserve what is left. Juana and Kino are returning home. They are all that is left of the family, as Kino learned too late what the word priority really meant. For all that Kino believed the pearl would afford him, no amount would have been enough to make-up for what he's lost. The loss of Coyotito was immeasurable; Kino and Juana's son was priceless.