The author isn't forgetting people who are hard of hearing; he's just not addressing them. It's a perfectly legitimate choice to make. It sucks for you. It also sucks for the people who don't read Clojure. You can't blame the author because he isn't catering to your needs.

I've made webcast videos before. It's not easy. I also didn't caption them because it's a hell of a lot of work. Sorry. You can ask, but you can't expect.