You can try adding the M4B extension to nullsoft directshow decoder play it. it still depends on various factors like audio encoding used, whether file is DRM protected, if its DRM protected then it may not play in WACUP
steps to open the config
Preferences -> plugins -> input -> click on Nullsoft directshow decoder in right pane -> click configure at bottom of preferences window. add the extension after adding ; .
ENSURE THAT you don't change, delete, mistype any extension in that box.
please post details about your expriment with this, this will help others finding similar answers.