What are the signs of a boy's voice changing?
About two years after puberty, your body begins to grow rapidly from your childhood size to your adult size. Among the many things that grow is your voice box and vocal cords. The voice box on an adult man is easy to spot, in America it is called the "Adam's apple." It is pronounced on men because of its size. The larger box accommodates the larger vocal cords.
Like a stringed instrument, the longer your vocal cords, the deeper your voice. Hence, one sign of your voice changing is the deepening tone of your voice. Unfortunately the change is rarely smooth. Your brain varies the pitch of your voice by tightening and loosening your vocal cords. However, with the increased length, it has to re-learn just how far to move the muscles controlling your vocal cords. At first it makes several mistakes causing the cords to bang into each other. Just like strings banging on a musical instrument, the result is a discordant squeak. It often happens when you are nervous (tense) and not thinking about keeping your voice pitched low, such as when you are talking to a girl. It is embarrassing, but it does disappear with time.