It is interesting stuff.
I remember years ago in the 1990's I used to travel to 'special' school on taxi's with the other kids. I remember a kid who was deaf. I think he was probably only about 5 or 6 years old. He had a brain implant. I think there was something on his head, similar to the suggested skull device Musk is talking about, and then small cables that went down to his ear. I think the device let him hear without having to use hearing aids.
So the basic technology as been around for 20+ years. It seems like Musk is expanding its applications. Probably with more efficient technology it is possible to do what he's saying. It's not that much of a big leap from what we have today.
Cochlear implant.
A friend of mine has had similar done when she was in her teens and had follow up surgery to update the hardware a few years back once she was around 30. Very interesting and impressive tech, but I think like many things in that area there's a lack of funding and drive for it to go as far as it should have done. With someone like Musk pushing similar tech it'll be exciting to see where things go.
I'm all for a bit of transhumanism personally, there's so many areas of life that could be improved upon for so many people.