Vocal cord cysts, or vocal fold cysts, are blisters filled with fluid on the vocal cords. The cords are structures that open for breathing, close for swallowing, and vibrate as air passes through them, producing sound. Cysts sometimes form when there is a blocked duct or a person is born with an abnormal mucous membrane. Cysts may cause cydysphonia, a condition where the vocal cords produce different tones at the same time, making speech hard to understand.