In the most recent issue of Romance Writers Report, one of the articles discusses the audio book market - specifically for authors. I will echo the sentiments in that article and say that if you're not taking advantage of that market as a writer, you're missing out.
Audio books may not be huge sellers, compared to print or ebooks, but they have a steady audience, and with the ease of downloading files today, I think that audience will only grow. Today, if you're signing a contract with a publisher, consider trying to keep your audio rights for yourself. In my experience, most small presses will take those rights but probably won't use them. I was lucky; Samhain never took my audio rights, so when the up-and-coming company Audio Lark was looking for new authors, I was able to sell my books to them. In the last year, Audio Lark has stopped producing audio books, but 4 of my books produced through them are up and available on Audible (the biggest seller of audio books, an affiliate of Amazon), and I still receive royalty payments from them every quarter.
Today, ACX provides indie authors with a super-easy way to produce their books in the audio market. All you have to do is sign up and post your project. You'll have to provide a script for potential narrators, and then you'll need to audition narrators and decide who sounds best. This is a crucial step! Make sure to take your time and choose someone whose voice matches the tone and content of your book. Also make sure the audition tape is professionally produced.
You have 2 choices when it comes to ACX: you can pay your narrator upfront (probably at least a few hundred dollars for a full-length novel), or you can opt for 50-50 royalty-sharing, which means you split all royalties but you pay nothing upfront.
Hearing your book spoken out loud is a crazy and wonderful experience. Being able to earn royalties on it is even better. If you haven't yet explored the possibility of turning your books into audio books, now's the time to do so! (And if you want to check out my audio books, here's my page at Audible. Please consider giving one a try - you can listen to free samples of each one!)