So one of my favorite game series is Portal and one of my favorite theories is the hard of hearing Chell theory. I got a few hard of hearing family members and so much of Chells personality and the story as a whole makes sense.
You can play the entire first portal game on mute. All of the story and game mechanics can be figured out without the need for audio. The second game, audio is important and required for understanding of certain things or mechanics, so I have a very solid explanation and theory to explain this.
Chell is completely unable to understand A.I. tone and voice, and I have gotten confirmation that the female AI voice is one of the hardest to comprehend. Glados, in the first game, uses a classic female AI voice before transitioning to a more human one, something that Chell would be unable to comprehend.
Once Glados gains a more human sounding voice and Wheatley, who has a normal voice, is introduced in the second game, then Chell is able to hear and follow instructions. She is still mute, though, or just refuses to talk.