What happens when what you see and hear do not match? You actually hear something that isn't there! Try it out to see for yourself.
McGurk effect
"The McGurk effect is a perceptual phenomenon that demonstrates an interaction between hearing and vision in speech perception. The illusion occurs when the auditory component of one sound is paired with the visual component of another sound, leading to the perception of a third sound. The visual information a person gets from seeing a person speak changes the way they hear the sound." - Wikipedia
