Every writer you speak to is going to have a different take on when to (or if you should) publish an audio book. Last year, the audio market topped $1 Billion. 35,000 authors shared that windfall
Every writer you speak to is going to have a different take on when to (or if you should) publish an audio book. I think it depends on the subject matter of the book. If the book you've written is timely and on the cutting edge, and you have the funds, I would suggest publishing e-book, audio book, trade paper, hardcover in that order.
If your funds are limited, I would suggest publishing e-book first, since currently 10 people will read an e-book before one reads trade paper. If your e-book goes well, I suggest publishing audio next. Last year, 35,000+ audio books were published to the tune of a $2.2 billion market. That averages out to approximately $38,000 per book (I think, and of course that does not mean each book will gross $38,000).
If both of those media are doing well, I would consider trade paper next, and probably not bother with hardcover, only due to the expense of hardcover copy. Of course, if your title is something that might be used as a reference, I would consider publishing hardcover in a limited printing.
Audio books and audio marketing have been mentioned in recent articles. This brings up a good point. Audio book teasers can be used as a fantastic marketing tool for your titles. The cost of an audio teaser is minimal, and not prohibitive. Audio teasers have been proven to out-market a standard Internet ad by 50 to 1. This means that 50 people will click on an audio teaser while only one will bother to click on a standard Internet ad that might take you to another standalone page. I will gladly supply you with additional information on audio book teaser marketing for the asking.
The link is just one example of a trailer produced by Hollywood Book Trailers. Mary's Diary, Jesus Through His Mother's eyes was edited by Moneysaver Editing AKA me.