Sound of Silence
Due to the Dursleys' treatment of him, Harry has grown up mute, and every time he tries to speak, he chokes. Because of that, he has learned BSL, ASL and created a magical notebook that lets him project his writing in front of other people to mimic a conversation.
He also has created a reverse legilimency spell that he sometimes uses with people he trusts very much and with animals. That's how he learns about Sirius being innocent, when he meets him as Padfoot. Sirius takes him to the Grimmauld place and accidentally gets in a screaming match with Walburga, whom Harry shuts up with "where are your manners, Black?", "you sound like a fishwife that got scammed 3 knuts for sardines" and "with the attitude you're displaying, your portrait is not worth for the canvas it was painted on to be used as toilet paper".