He's also not used to using his voice. With his inherent powers, a single whisper from Black Bolt can level Thor. A casual word could flatten a skyscraper. Since a very young age, he's learned to control his voice, to not even grunt or exhale audibly, but in this series he will be trapped in a prison where his powers don't work. His few lines of dialogue should be slightly broken - authoritative, yeah, but still spoken by someone who's not used to actually speaking.