https://www.aboutamazon.com/news/entertainment/audible-amazon-music-unlimited-subscription

Unlimited subscribers in the U.S., UK, and Canada can enjoy one book a month, including a selection of over a million audiobooks in the U.S., plus 100 million HD songs and top podcasts.

Amazon Music Unlimited subscribers in the U.S., UK, and Canada can now listen to one audiobook a month from Audible's unparalleled library of premium audio storytelling content, including a U.S. catalog of more than 1 million of the most popular and enduring audiobooks in the world. The addition of the Audible catalog makes Amazon Music Unlimited the premier destination for audio entertainment—now with access to more than 100 million songs in HD audio, the most top podcasts ad-free, and the largest catalog of audiobooks—including music and audiobooks in spatial audio.

Starting today, Amazon Music Unlimited individual plan subscribers, and primary account holders to the family plan, can listen to one audiobook at a time—of any length—per month with their subscription. Customers can stream at their convenience and can continue listening to their monthly title after the next billing cycle begins, or select a new one. While Amazon Music offers access to one Audible title per month, customers who want more beyond their monthly listen can subscribe to an Audible membership or purchase titles a la carte directly from the Audible app.