The new Audible feature ties into Amazon's Whispersync technology, which automatically syncs progress between the Kindle and Audible versions of a book. The feature allows Amazon customers to read a book at home and then listen from where they left off when they open the Audible version of the book. In order for his feature to work, customers must own both the Kindle and Audible version of the book title.
Other changes in Kindle 4.3 for iOS version include the ability to download a collection of books, the addition of a sleep timer and new narration speeds up to 2x. Amazon also improved its X-Ray search feature and improved overall performance and stability of the app.
Kindle for iOS is a free app that can be downloaded from the App Store. [Direct Link]