From the audible website: http://audible.custhelp.com/app/answ...le-for-android
Devices without the Android Market are not officially supported for use with the Audible for Android application. If you would like to try installing a beta version of the application on your device, you may do so by visiting our Google Group at the following link.
Please Note: All beta application assistance and support related questions must be directed to the Google Group forum. Our Customer Care team is not able to provide support to users of the beta application.
Audible for Android Google Group