a completed audio book

I’ve completed recording a project that includes not only spoken word but also numerous selections of music, currently as wav files. Since the total duration for all 39 chapters is 12 hours, I’d like to know what sort of file format (or compression) would be best, without losing audio quality.

ACX (who handle audiobook submissions for Amazon) specify that each chapter must be a separate file, no longer than 120 minutes. Full details of their specification can be found here: https://www.acx.com/help/acx-audio-submission-requirements/201456300

You backup copy should be in (lossless) WAV or FLAC format.
The version that you send to the publisher should be in whatever format they specify. ACX specify “192kbps or higher MP3, Constant Bit Rate (CBR) at 44.1 kHz”.

As Steve above, your own Edit Masters should be in a perfect, uncompressed format. You can’t edit or change an MP3 without sound damage. ACX’s request of mono (not stereo) format and their requirement of at least 192 quality MP3 is so they can resample your work for different products and services without serious loss of quality.

not only spoken word but also numerous selections of music

You didn’t produce an audiobook. You produced radio theater. ACX doesn’t like music in the performances. At most you can put in a little chapter intro or outro music, but that’s it. If you look over the instructions and comments, they put great stress on you being able to certify ownership and copyright of the work you’re reading. As of the sickness, it got worse. Anyone has to be able to buy the book on Amazon (in any form) before you submit. As an extension of that, you have to be able to certify the copyrights, ownership, permission, and performance of every piece of music in the book.

Audacity publishes ACX Check which is a simulation of the first level of ACX acceptance testing when you submit. That tests basics such as voice loudness and background noise volume. Did you pass that OK? It’s not unusual for home readers to fail background noise (Room Tone). Getting background noise a thousand times quieter than your voice isn’t easy.


what if the music is either produced by yourself or it is your friend’s who gives you ownership? That must be okay

Music adds a lot of dynamic, kind of turns it into a play and makes it feel alive