Why can watching a mouth move change the sound you hear?

Your ears report one thing, your eyes overrule them, and you never notice the swap.

Hearing is not just an ear job. The brain fuses what you see on a speaker's lips with what you hear, and when the two conflict it can invent a third sound that nobody actually made. Watch a mouth say "ga" while the audio plays "ba" and most people hear "da" - a blend that was never spoken. The eyes quietly edit the ears.

Watch a dubbed clip where the lips and the audio do not match, and the words can land slightly off - your eyes patch the sound from the mouth, not from what was really played.

What you see a mouth do can rewrite the sound you think you heard, without you noticing the edit.

Your senses do not just report the world, they negotiate it. Knowing sight can overwrite sound makes you slower to trust a single sense as the whole truth.

McGurk = mouth + ear merge. Lips say one thing, ears another, the brain serves a third.

Learn the idea and practice English at the same time.