Saying the world is designed for people with hearing is causally inaccurate. Hearing evolved because sound is a useful way to perceive the world, the same way vision evolved in the spectrum it did because those are the strongest frequencies of sunlight. And more generally, I don't see many things in my everyday life with exclusively audio feedback. Usually anything designed with an audio cue just uses it to reinforce a visual one. Things we are interested in (animals, cars, etc) make noise, and we have evolved hearing in response to that. Without hearing, you have no way to be notified about things outside your field of view. This is not something we've designed about the world, it's just how that sense works.
The world was dark half the day for much of human history. Communicating in the dark is clearly easier with noise than signing or passing tactile writing around. It's not like we just invented noise for the fun of it, it's just a better medium in most situations.
Well, most of human activity, then and now, is conducted in a lighted environment. Regardless, it is not clear to me how the reasons for vocal/aural communication arising as the dominant mode is at all relevant here.
They probably meant "the human world", cities, etc. For example in the subway, the warning for the closing of doors used to be only audio, now you can see more modern ones that have both audio and visual cues that doors are about to close. In the same way, when crossing at a traffic light, there used to be only visual cues. In some cities now you have places that also have audio cues for people that need it.
