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.