The last line of "The Pearl" is referring to the song of the pearl that at one point was great and joyful. This music however turned dark when it brought more evil into Kino, Juana, and Coyotito's lives. The pearl brought more illness, greed, and desperation in their long days to the point where they had to take things into their own hands to defend their family.
The pearl was not only a symbol for the terrible forces of the world. In a was it was also a representation of hope, opportunity, and the ability to better their lives.Kino said that, with the money he would get for selling the pearl he would get him and his wife married, send Coyotito to school so he will learn to read and write, buy new clothes, and purchase a rifle. The pearl was the door that would open all of these opportunities and more.
There was a definite dark side to the pearl that vastly overshadowed the light. While there were good chances to arrived on that light side, the black cloud quickly engulfed Kino and Juana. Their lives were sent on a roller coaster. They discovered what the pearl was doing to their family, so at the end of the book they throw it back to the sea. This bring us to the line that we are interpreting. The music of the pearl is turning to a whisper and fading away as it drifted to the bottom of the ocean along with the evil that came to the land with it.