Not sure if it will work with the WMA files at Audible.com but WMA Encode/Decode by X-Filez will transcode wma files to mp3 and claims to work with DRM protected clips assuming you have a license for your player to be able to play them.

I've used this on unencrypted clips and it works in > realtime. Worth a shot.

http://www.x-filez.com/aedtoolspage.htm

-Kit