Th supraglottis is the part of the throat where speech originates. It lies just above the larynx (voice box) and below the epiglottis (flap that covers the trachea [windpipe] during swallowing so food doesn't get in the lungs). It includes the upper part of the larynx, the vocal cords, and several other appendages that contribute to speech (false vocal cords, ventricles, aryepiglottic folds, and arytenoids).
Speech originates in the speech center of the brain, located in the folds of the cerebral cortex. A nerve impulse (signal) of what we want to say is sent to the vocal cords. Air is inhaled into the lungs creating pressure. As we exhale, air passes over the vocal cords in the larynx, stimulating them to vibrate. We then form those fibrations into words by how we move our tongue in our mouth and against our teeth, by how wide we open our jaws, and the by the shape of the lips (articulation).
The signal then races back to the speech center in the brain, tells it what we said, and interprets it for us. This all happens in a split second and often without much conscious thought on our part.