Booksonboard also sells audiobooks...they are mp3 downloads. The public library audiobooks use a version of Overdrive for audio files that you would have to download to your computer. My library has wma or mp3 format. Audible has "apps" for mobile devices if you are planning to try that one.
Audiobooks are a lot more expensive than ebooks. You might want to try out "text to speech" instead with an app like Mantano Reader. Haven't tried it myself but I understand that some persons find it surprisingly good.
Quote from http://jasonrenshaw.typepad.com/jaso...ture-kids.html
One function I particularly enjoyed, however, was the TTS (Text to Speech) option, which when activated basically reads the text to you out loud. It does have that flat, somewhat expressionless monotone you expect from a computer-generated TTS function, but it was much better than I thought it would be and for the reader who enjoys the aural supplement I think it does a wonderful job.