To produce a sound we need air, sound source and resonator (mouth and nose). We inhale and then while exhaling our muscles around the ribs squeeze and we release air through the resonator. Air that goes from lungs also goes through vocal cords and takes the sound they produce by stretching and vibrating.
   Different positions of vocal cords can change the sound, for example, if they really close (almost touching each other) we speak in a whisper. Pitch and loudness depends on a person, reasons for that could be longer, thicker or more tense than other vocal folds. Moreover, we have different sizes and shapes of our throats, mouths and nostrils.