Human vocal folds in action - youtube video
Human speech sounds come in two flavors: voiced and unvoiced. Voiced speech sounds are generated when the vocal folds produce a rapid click train, the so called "glottal pulse train", which then resonates around the vocal tract. This youtube video of a human laryngoscopy procedures shows the vibrating vocal folds during vocalization. Note the muscular apparatus which can open the vocal folds wide when the subject takes a deep breath, or vary the amount of tension on them to change the voice pitch.