Girls also undergo a voice change,buy it is not as pronounced as in boys. the larynx in the throat is responsible for the voice and the vocal cords are present inside it . in children, the vocal cords are short,so that the voice is rather shrill. during puberty, the larynx grows rapidly in boys and one can clearly see it,the 'Adam's apple', in the centre of the throat, the vocal cords enlarge and become thicker, and the voice becomes deeper, the muscles of the vocal cords take time to adjust to the changes. till then, the voice'!jumps' between high and low tones and is either shrill or deep. 
Epiglottis: Air from the lungs flows along the vocal cords . if these are under tension,they produce a sound.