I think when playing audible files, you need to be able to connect to the internet so WMP can download licenses from Audible.com. Is the audible software suite installed on your carputer? Have you installed the audible kit for WMP? Are you connected? Else WMP will just wait for a license that never comes. Sometimes when launching audible files directly from WMP i've gotten a login window from audible to continue... you might not see this with the SD overlay. Try it in windowed mode.
Have you thought about ripping your audible files to mp3? That's the way I do it. It's easier for me and my portables that don't support the audible format.