What does the monkey 音楽 box mean?

When Erik/The Phantom was in the fair as the Devil's Child, he owned the monkey as his only toy. When Christine went to his lair she saw the monkey down there and associated it with him. Near the end of the movie, after The Phantom lets Christine and Raoul go, he pulls the monkey out and sings "Masquerade.....hide your face so the world will never find you" I think that the monkey was his one thing from his childhood that was good and he wanted to remember the good from his childhood.