Nick's voice is changing. It is one of the many developments that happen to both girls and boys when they reach puberty: a boy’s voice gets deeper than a girl's.
At puberty, boys' bodies begin producing a lot of the hormone testosterone, which causes changes in several parts of the body, including the voice. For starters, a boy's larynx, also known as the voice box, grows bigger, which causes your voice to get deeper.
Along with all the other changes in your body, you might notice that your throat area looks a little different. For boys, when the larynx grows bigger, part of it sticks out in the part of the neck at the front of the throat and forms the Adam's apple. For girls, the larynx also grows bigger but not as much as a boy's. That is why girls don't have Adam's apples.
While your body is getting used to these changes, your voice can be difficult to control. A guy’s voice "cracks" or "breaks" because his body is getting used to the changing size of his larynx. Fortunately, the cracking and breaking is only temporary. It usually lasts no longer than a few months.
Generally, a boy's voice will start to change somewhere between the ages of 11 and 15 - although it can be earlier or later for some.