Home reading audiobooks is not for the easily frightened.
I hope to goodness you have WAV or Audacity Projects of each raw chapter of the read. We can’t take effects, corrections or filters out of a production. Once you master, that’s the end of the world.
There is a published ACX Mastering process.
https://forum.audacityteam.org/t/audiobook-mastering-version-4/45908/1
Please note it starts with a raw, uncorrected reading and if you read in a quiet, echo free room, that’s all you need.
Noise Gate and stiff Noise Reduction are not recommended. If you believe ACX, they are forbidden. They take a hard line on this stuff.
So, do you or your presenter have Audacity Projects or WAVes of the original, raw, unprocessed reading?
Koz