As Audible is not available on Sonos in your Country, I would suggest placing your feature request with Amazon aswell, just so that they are aware that their customers are requesting this. An approach from both sides will show the demand for it and if the demand/infrastructure is there the two companies may then come together to consider this request - its probably worth making both sides aware.

Came here to add to this. It's rather sad that this still isn't supported.

Have you made the request to Amazon?

Yeah, they support Sonos

Amazon controls where their services are allowed in the Sonos app. They are required to give Sonos legal authorization before Sonos is allowed to carry a connection. It seems like Amazon has yet to do so for Audible in this location…I wonder if there are local legal issues that make this more challenging. 

I checked the support page on Audible.in first and it says they support Sonos, so I'm not sure who's dropping the ball here.

It's irritating when people are caught in the middle of two companies that have only partially figured out how to work together.