Update (partial success): after visiting Amazon’s ‘Manage apps & services with data access’ page and removing items related to Sonos, I saw later that the Audible service had automatically dropped off the list in the Android app (not immediately, perhaps a half hour later -- as I had already done those steps before, including resetting the Android app itself, multiple times).  Unfortunately I still was unable to add it again directly.  However when I tried sending audio from the Audible app to a Sonos speaker I then got a web prompt to grant Audible access to Sonos.  (I had gotten this earlier too but the “Accept” link was disabled -- this time it worked.)  At that point I was able to play Audible from my phone to Sonos.  However… Audible still does not show up in the Sonos Android app.  It DOES however appear in the Windows app!  Three instances of it actually -- only one of which worked so I deleted the other two.  So now I can play Audible through Sonos either directly from my phone’s Audible app, or from the Sonos Windows app, but not through the Sonos Android app.  So far I have not had to factory reset any of my speakers.  But ugh, what a mess Sonos is.