Joanna Penn writes this back in March, but it's a good look at audio books. She interviewed J. Daniel Sawyer about his experiences producing audio books:
Your book is not just a physical book or an ebook. There are plenty of other subsidiary rights that you can exploit and audiobooks are high on the list because of the rise in popularity of listening during commutes or workouts, and the increased penetration of smartphones. In today’s interview, we explore how you can get into this market.
My experience with audiobooks is very limited. I listened to all of 'World War Z' read out by various actors, and it was great; I listened to Carl Sagan reading from 'The Demon Haunted World' and it made me sad.
There's a podcast of the interview, as well as a transcript, so if you're interested in the process of producing audiobooks it's a good place to start. Take, for example:
Essentially, you will always make mistakes while reading. A single read when you’re really good will take about 4: 1 editing time. So for every finished hour of audio, you need at least 3 or 4 hours of production. If you’re just starting out it can be more like 10 hours production to 1 of finished audio. This is why it can be expensive to produce good quality audiobooks.
Read it here: http://www.thecreativepenn.com/2013/03/06/audiobook/