what explanation can be given of why kino heares the " song of evil " when the priest comes to their house?

The priest enters Kino's house and the Song of Evil follows close behind him. Kino is uncertain of who brought it, but he can hear the evil song softly behind the priest's questions about the pearl." The song of evil seems to come whenever there is bad energy. It is sort of an ominous foreshadowing that has to do with mans' disease of greed and power. The church turns out to be no different than the Pearl merchants and speculators. The Priest says that he knows Kino "will do good things" with his new fortune. The subtext is that the Church expects its share.

It's used as foreshadowing; the song symbolizes the eveil to come.