You will be able to make your book an audio book at any time, either during upload or in the future as a revision to your book. All you need is a single MP3 file that has you or someone else reading your book. Be sure to create about 3 second time gaps in between each page to make it easier to tag page turns.
Once you add your MP3 file to your book, you will have 2 options, Connected Audio or Disconnected Audio.
Connected Experience
This is when you want to create a follow along reading experience, so that when the reader is alerted to turn the page...