isn't it the responsibility of the dominant person to make sure they hear it when spoken, or to arrange some other kind of signal if the person being dominated can't speak

Yes. And to check in with the submissive to make sure everything's all right. And to be very, very aware of everything that's going on.

I'm a sub and I'm fucking tired of rapists and rape-cheerleaders claiming that consent can possibly be murky in a non-abusive BDSM relationship. It can't. It's much clearer than it is in most vanilla relationships, in fact.