The McGurk Effect is a multisensory phenomenon in which inconsistent audio and visual stimuli are paired to create the misperception of a third sound. For example, when presented with a video of someone saying “ga” simultaneously with audio of saying “ba,” many people actually hear the syllable “da.” The McGurk Effect was first described by McGurk and MacDonald in 1976 and has been replicated many times. It remains a robust illusion, with factors such as sex, familiarity, being the speaker, whether there is a lag in stimulus presentation, and several others either strengthening, weakening, or not altering the effect. The McGurk Effect is an example of how top-down processing influences the actual stimuli – otherwise, identical auditory information would always sound identical regardless of the visual information being presented. In other words, what you hear isn’t always what you see!
McGurk H., MacDonald J. (1976). Hearing lips and seeing voices. Nature 264, 746–748. https://doi.org/10.1038/264746a0
And here is a good video demonstrating it: https://www.youtube.com/watch?v=G-lN8vWm3m0