I'm having the same problem on one movie only: Rocketman. Everything else on Amazon is fine. Well, except for the unadjustable tiny subtitles, an inexcusable problem if you ask me. I've tried changing the language but it doesn't seem to accept that command.
If you have seen this solution already, I apologize.
This solution worked:
1) Start your film then hit pause.
2) You should see two icons: text (subtitles) and audio (languages). Select Audio & Languages.
I was having trouble with the movie "Yesterday." I paused it, chose the English (UK) without the horrible Audio Description and now I can watch the movie. You are right, the Audio Description from Amazon does not work on the Roku, but now at least I can watch the movie.
I agree, the app is defective. What made things work for me was starting the movie, then pausing the movie, and only then could I get at the options. If you've done this (start and pause), then perhaps we have a different setup or perhaps your movie has no option for another English language. I wish I had some other workaround.