I have written back to ACX to ask if they can tell me what I have to do
You may not get a reply. Their job is to insure high quality work for their platform, not do detailed analysis and servicing. Most of the world is trying to read for audiobooks and their quality control offices are overloaded.
Problem is I actually used Reaper and so I only have finished files in MP3 format???
Do you have the unfinished files in WAV? Reading, processing and submitting one single sound file per chapter is a very serious new user error. Every time you make, change or correct an MP3, the sound quality gets a little worse and you can’t stop it.
re-record a 15 hour book
ACX has provision for you to submit a brief sound test rather than arrive one day with a complete book.
This isn’t the whole story. They don’t actually publish specs for this. The spells are actually those for publishing a book summery. Between one and five minutes of typical work. I need to drop for a while.
It’s notes one and three of these publication instructions.
That and of course it has to pass technical conformance.
Koz