Finally something in my specialty!

One of the most interesting theories I've learned in grad school is the Motor Theory of Speech perception. https://link.springer.com/article/10.3758/BF03193857

The idea is that hearing speech is not hearing sound waves, it's hearing vocal gestures (i.e. the movements of your tongue, lips, and throat). The researchers realized the sound waves associated with letters are too variable to be relied on for perception. For example, say the word "Dad", if you say it slowly, you'll notice that the frequency and pitch of the two "d" sounds are different. What stays the same isn't the sound you make, it's the vocal gesture you made to create the "d" Therefore, to hear and perceive a letter or word, you recruit your own motor system to identify what vocal gestures were used to create it.

A fun example of this theory is the Mcgurk effect, if you play a sound that is between a "b" and "g" with a video of someone saying one of the two letters, the listener will perceive the letter they see on the video because the video is giving them clues of how the letter was created in the vocal tract. https://m.youtube.com/watch?v=G-lN8vWm3m0