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/).