These files have no commercial value (to the best of our knowledge, and intent). These files are being used in the development of a FFmpeg plugin (capable of handling Audible content). See https://ffmpeg.org/pipermail/ffmpeg-devel/2015-July/176109.html for more details. Hosting of sample files (for development, vendor interoperability, and testing purposes) is a common practice (e.g. http://samples.ffmpeg.org/audible/).