The Sonos app can be added to as many devices as you like so your wife can add the app to her phone. To listen to her audible books just add her audible account as a service.

Settings / services & voice / scroll to the end of music & content / add a service / scroll down to audible and follow the instructions

Note anyone who has access your Sonos system will now have access to audible, this may be a good or bad thing.

My wife needs to register a different account, or she must use my acc? 

It isn’t really your account.  Think of it as the system’s account.  There can be only one Sonos account per system.  As @Ralpfocus said, service accounts for any and all users can be added to that account.

Same Sonos account.  But Sonos supports multiple accounts for each service, so she can add her services and set them as the default on her controller.  See: https://support.sonos.com/s/article/3624?language=en_US

Thank you, I'll try!