The signs of adding audiobook was first spotted by Google watchers.It discovered an empty page in the help section of Google Play. It concludes that Google is going to add "Audio books." to Google Play.
Google watchers also observed that Google is registering a number of domains related to Google play, which suggest the expansion of Play stores. From those registered domains it seems like Google is going to include magazines, newspapers and TV shows in Play Store.
Adding these kinds of content reminds us Apple’s iTunes store which also offers same kinds of service and products.