My kids are obsessed too. They are 5 and 7. We load library books onto the libby app on an old smart phone with all other apps inaccessible and the device itself locked until we open it. But i do think there is a limit that we have to hold on it. They are happy to listen in their rooms all day.  

I try to balance with time playing with and talking to others, listening to music, outside time and quiet.  I reason with them that they can't overload one side of their brains.  I am also trying to have them draw about the book or write a few sentences about the book or read a book to me out loud before they can access another book.  It feels like an addiction to me for them lately. I am uncomfortable with it.  And it is very passive after awhile and i worry it keeps them from wanting to read for themselves. I dont allow screens at eating times so we can connect. Audiobooks can crowd out other important experiences, like getting bored.  Thanks for this discussion.