According to The Association of American Publishers, the national trade association of the U.S. book publishing industry:
"March 17, 2010, New York, NY – E-books and downloadable audio books continue to grow in popularity according to the January 2011 sales report of the Association of American Publishers. Figures for the first month of the new year show that E-book net sales increased by 115.8% vs January 2010 (from $32.4 Million to $69.9M). Sales of Downloadable Audio Books also rose by 8.8% vs the previous year ($6.0M to $6.5M). As AAP reported last month http://tiny.cc/obolv in its December 2010 monthly report and full 2010 analysis, E-book sales have increased annually and significantly in all nine years of tracking the category."
I know for my ebooks sales over the last decade, there has been a nice increase, especially since Kindle came along, and now Smashwords, which has given me the opportunity to pulish more of my books as e-books.
I'm sure e-book sales will continue to go up and up. I would imagne the ability to download mp3 digital audio books will continue to increase audio book sales also.