No such feature.
If you have audiobooks from (for example) iTunes, Audible, Librivox, a library or on MP3, you can play them on iPad, but that involves buying, borrowing or getting freebies from sites such as Librivox.
If you're looking for books read via audio and don't want to pay or borrow, you might check out Kindle e-ink devices that offer a text-to-speech function. A computerized voice will read books if the publisher allows such use. It sounds like crap to me, but some people don't mind.
