I have always thought Audible.es was/is accessible via the Sonos App? … I do see the folder in my App (see attached screenshot). I cannot test the link however, as my audiobooks are stored in a U.K. account.

Is your Sonos profile ‘country’ set to Spain here in your online Sonos profile?…

https://www.sonos.com/myaccount/user/profile/

There could be geo-blocking by Audible/Amazon. They have to authorize Sonos to carry the content there, and may not have yet, even if it’s available already in the Audible app. 

The list of countries where Audible is available on Sonos is huge.  Spain is one of the few territories where it isn't.   You're fine if you live in Afghanistan or Vanuatu..... Bizarre. 

Legal situations from country to country can be complex. Navigating those is not a fun thing to do, I suppose that’s one reason lawyers make the large salaries they often command.