Final answer:
Air forced through the vocal cords causes them to vibrate, producing sound. The pitch of the voice changes when the tension on the vocal cords changes. The volume of the voice increases with increased airflow from the lungs.
Step-by-step explanation:
When air is forced through the vocal cords, they begin to vibrate. This vibration produces sound. As air travels up and past the vocal cords, it causes them to vibrate. This vibration escapes the mouth along with puffs of air as sound. The voice changes in pitch when the muscles of the larynx relax or tighten, changing the tension on the vocal cords. Additionally, the voice becomes louder when air flow from the lungs increases, making the amplitude of the sound pressure wave greater.