One of the motifs that Steinbeck uses to illustrate the theme of good vs. evil is music. When Kino awakens, he is filled with a good feeling and he refers to the music he hears during this feeling as the Song of the Family. The scorpion is accompanied by the Song of Evil. The Song of the Family cries "plaintively" in response to the Song of Evil. So, there is the sense that the songs, and/or fores of good and evil, are competing against one another.
Everything in Kino's world and culture corresponds with music. This gives the notion of good vs. evil artistic, abstract, and universal qualities. This music can be personal or it can relate to Kino's entire culture and ancestors. Thus, the battle of good vs. evil is something universal, spiritual, and something that has always been part of their culture:
Kino's people had sung of everything that happened or existed. They had made songs to the fishes, to the sea in anger and to the sea in calm, to the light and the dark and the sun and the moon, and the songs were all in Kino and in his people - every song that had ever been made, even the ones forgotten.
Just as there are songs for both good and evil, the pearl itself contains the potential for good and evil. It represents hope but contains the potential for sin and destruction. The mix of old and new (Catholic) religions shows how this theme is common to many religious philosophies. This "singing" or praying is a way that Kino's people try to make sense of the world. Kino is confused when he hears competing songs of good and evil when he speaks of the pearl. (This is initially noticeably when he first speaks with the priest.) The ancient magic, the Catholic Church, the songs themselves are all ways that Kino's people try to understand the potential for good or evil. One of Kino's tragic flaws is that he initially fails to associated evil intuition/music with the pearl. Of course, it is not the pearl itself that contains evil. It is the pearl's monetary value that brings out the greed and thus, the evil thoughts of anyone who covets it.