This problem of the "Enable airpod case sounds" being disactivated has been plaguing me too!
This worked for me: I started playing around in the settings, changing things one by one to see if the case sound option would activate. I changed "Connect to iPhone" from "Automatic" to "If it was the last device used" - nothing. Then I changed "Microphone" from "Automatic" to "Only left." And boom, when I went back to the main page of airpod settings, I was able to disactivate the case sounds.
Maybe the trick is not to do exactly as I did, but to play around with the settings (maybe especially ones you haven't touched before?) until that one activates...unsure what conclusion to draw, just so relieved!!
[Edited by Moderator]