There are several songs in The Pearl. Steinbeck envisioned this book becoming a movie, so you can think of these songs as background music creating mood:
The Song of the Family: this accompanies gentle, peaceful domestic scenes, often outside by the waves, as the family goes about its cycle of work. It is a calming song.
The Song of Evil: this song occurs at moments of danger, such as when the family sees the scorpion threatening their baby. It is a "savage" melody.
The Song of the Enemy: this is very similar to the Song of Evil: it comes into Kino's head as he has to fight the scorpion.
The Song of the Undersea: this is the song of all of Kino's people and is about everything that has ever happened. It is sung to the fishes, the sun, the moon, and the sea.
The Song of the Pearl that Might Be: this is the song of hoping to find a pearl. Finding a pearl is a dream, so this a song of the unlikely, but it is still a potent song.