Thank you. This was already solved and can closed.
Here is what I wrote on Monday, 9-7-20, via e-mail to Sonos:
"Thank you for your reply . That method didn’t work and I’d already tried it before I wrote to Sonos. I had also contacted Audible telephone support about this same problem. This last Saturday a knowledgeable Audible telephone support specialist said that Audible already knew about the problem, and that it would be impossible to play Audible’s new Plus titles, expect by streaming them through a browser at the Audible site.
Then, literally yesterday, Audible released an update to their iOS app. Although their previous release had a significant problem over which I had to telephone them to solve, I tried the update of yesterday. The update succeeded.
As of a few minutes ago I’m able to see the Plus titles inside the Audible app, whereas before that was not possible. But your Sonos app does not show Audible’s Plus titles at this writing.
This case can be closed as imagine Sonos might eventually work out its’ integration with Audible, and meanwhile I’ve the ‘workaround’ Audible made possible only yesterday.
Thank you.
Moderator Note: Modified in accordance with the community guidelines.