Tantor Media will be releasing the Audio version of Into the Abyss, narrated by Stephen Bel Davies on April 25th, 2017.
I would expect on that date, or shortly thereafter it will be available on Audible and other audio book stores.
It may take a bit of time for it to link up with the kindle version. No idea how that works when a third party is producing the audio version.
Volume II should be available about a month or so later.
Volume III? Well that depends on the sales of Volumes I and II in audio format. :)
T-A-G