One thing this lark has shown me is why the voice of a narrator varies ever so slightly between sections of a chapter. It’s because of re-recording. I’ve found that it’s more likely I’ll get away with it if there’s dialogue in between my narration.
I’m in the final stages of fettling my recordings. For reasons that I don’t understand some sound files (each chapter is a sound file) that previously passed the ACX plugin test were now failing it. All the effects I’d used before to make things right now didn’t work. After taking a break to prevent myself putting a fist through the laptop screen, 
This worked for me:
PROCESS:
These instructions are in short-form: Location > Tool: Options > OK
Select the whole reading or chapter by clicking just right of the up arrow button (on the left).
Effect > Filter Curve > Manage > Factory Presets > Low roll-off for speech > OK.
Effect > RMS Normalize: Target RMS Level -20dB > OK.
Effect > Limiter: Soft Limit, 0, 0, -3.5dB, 10, No > OK.
Analyze > ACX-Check.
Once the Audacity aup files have satisfied the ACX check, they can be converted to MP3 format. Create a folder on your desktop labelled MP3. The conversion is easily done in the latest version of Audacity by clicking:
File > Export > Export as MP3. Send the files to your MP3 folder. Remember, opening credits and closing credits (The End) need to be in separate files. The opening credits need to be the same as those on your eBook.
While doing this, a newsletter came in from ACX. It told of one new and one recent quality control tests. The latest is called Audio Lab, whilst Audio Analysis has been around for a few months. So far as I can see, Audio Lab is simply an expansion of the ACX plugin to be used when you’re finished, whilst Audio Analysis is aimed at those unsure of settings and their microphone for them to check what they’re recorded so far.
* Audio Lab – Sound Check: Audio Lab Launches on ACX
* Audio Analysis – ACX Audio Analysis Tool FAQ’s
As you can tell from my recent posts, narrating, editing and mastering an audiobook takes a long time and is exasperating.
My recorded chapters have passed the ACX check, the Audio Lab and Audio Analysis without any problems, but they could still fail at the Human Quality Control. There are hundreds of ways of mastering what you’ve recorded and yet, there’s this dire warning on the ACX website:
Human Quality Control at ACX (the theatrical test after you pass ACX-Check technical test) does not like heavy processing. You should be as gentle as you can with as few corrections as possible. Don’t even think of submitting readings that sound like a bad cellphone, speaking into a wineglass or reading in a bathroom.
The AudioBook metaphor is listening to someone telling you a story over cups of tea. Anything that distracts from that ideal should be avoided.
They don’t define what “few corrections” means! How few?