Characters that can speak but choose not to/aren’t shown doing so

Soundwave (Transformers Prime): Took a vow of silence and only spoke to mock the Autobots

Link (Legend of Zelda): Speaks to NPCs and stuff but it isn’t shown and in some instances is a choice such as Pre Calamity Link in Breath of the Wild

Toto (The Wizard of Oz novels): Animals in Oz can speak even if brought from outside of it. When someone asked about it, Toto responded along the lines of “because I didn’t have anything to say”