One useful feature from some other audio book players is a "Replay" button that automatically replays the last 20 or 30 seconds of audio.  It is an easy way to go back and catch that idea that was missed during a moment of distraction or when something didn't sink in the first time.  The button could either be just a standard 30 second replay (like in reader for the Audible audiobook service) or could be set up with a customizable default.