In Audible's infinite wisdom they have made all upgrades to the media player filter or their main program block the ability of db to work. You need to find a version of AudibleMediaPlayerFilter.exe from 6-6-2006 or older. Reinstall it over what ever you have. Then you will be able to convert again with dbpoweramp.

*this is assuming you have already configured your directshow to see .aa files. Works with old and new versions of db.