What are vocal cords and how do they help to produce sounds in human beings? How do muscles attached to vocal cords regulate sound?

The vocal cords (also called vocal folds) are two bands of smooth muscle tissue found in the larynx (voice box). The vocal cords vibrate and air passes through the cords from the lungs to produce the sound of your voice. 
The vocal folds produce sound when they come together and then vibrate as air passes through them during exhalation of air from the lungs. This vibration produces the sound wave for your voice. In order for the sound to be clear and not raspy or hoarse, the vocal folds must vibrate together symmetrically and regularly.
Muscles attached to the vocal cords may make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different from that when these are loose and thick.
When we speak muscles in the larynx bring the vocal cords together. As the air rushes from our lungs out through the larynx, the from portion of the vocal cords vibrate, producing a sound. The upper portion of our throat then modifies this sound to produce speech.