The other day I reported that Google is officially showing if a book supports audibooks and if so, they will link to the place you can listen to the audibook - typically on Audible.
After I posted that, the king of schema and markup, Aaron Bradley posted on Twitter showing him testing the AudioBookFormat schema to show Google may be picking it up. He shared this screen shot of the output:
Also, on the books developer page around schema there is a link to fill out your interest in participating in this. One of those options are Audio Books.
So if you sell audiobooks, it might make sense for you to look into this.
Forum discussion at Twitter.