voices are the result of vibrations over the muscles that we normally call the vocal chords. There are lots of properties that can affect these vibrations. Size of the vocal chords is one way to think of it. A guitar or other stringed instrument is a good example. some strings are thicker than others and this affects the sound, but so does the tension in each individual string. Our voices are a combination of these factors. One of the reasons why your voice gets weird when you are sick is because these muscles get irritated and swell and that changes the way they vibrate.
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