Many years ago, I took a beginners’ course in German Sign Language at the Volkshochschule. (I dropped out after, I think, two semesters, because I was a bit frustrated with the pace of the course.)
Our instructor was deaf, but he could speak well and he could lip-read well, so we communicated with him by speaking.
Except for one smart-aleck show-off who would simply mouthe her words at him; he could understand her because when you’re lip-reading, you don’t care whether the vocal cords are vibrating or not, but everyone else was rather annoyed. (Hearing an exchange along the lines of “…” - “Yes, you’re right” isn’t very helpful to anyone else!).