What is actually changing
The larynx, often called the voice box, grows during puberty. The vocal folds become longer and thicker, and the spaces in the throat, mouth, and chest change how sound resonates. Coordination has to adjust while all of this is happening.
That is why a voice may alternate between higher and lower notes, crack during a sentence, feel tired after a lot of talking, or sound different from one week to the next. Girls’ voices also generally lower during puberty, although the change is often less dramatic.