It's something that's bugged me for a while. Bumblebee doesn't have a mouth. He has nothing that he could speak with. If he was mute he should have a mouth. Unless Ratchet sealed it up or something. It's strange, since Bee was able to talk at some point. Maybe he had an injury like Bane in TDKR, but had his mouth severely injured which led to him being mute, and wears something to cover up his mouth/ lower face, similar to how Prime has that face mask. Except, when Bee "talks" all we see is his head moving, and all we hear is a clear and loud beep.It's not like we hear some muffling.