I've QC'd a lot of audiobook files over the past year. Same mistakes come up again and again. Here's what's actually causing your rejects:
1. RMS measured across the whole file, not per chapter.
ACX requires -18 to -23 dB RMS, but they measure chapter by chapter. One quiet intro chapter can drag your average into "pass" territory while individual chapters fail. Check every chapter separately.
2. Noise floor measured in the wrong place
-60 dB or quieter. Most people measure at the very start of the file, but ACX measures in any silence segment throughout. That breath before a paragraph, that pause mid-sentence. Your noise floor needs to hold everywhere.
3. True Peak vs. Peak. These are not the same thing
ACX requires -3 dBTP (True Peak). Your DAW's peak meter is lying to you slightly. Inter-sample peaks can clip above 0 dBFS even when your meter shows -3. You need a True Peak meter (most modern limiters show this, Fabfilter L2, iZotope, Waves L2 all do).
4. Room tone doesn't match between chapters
Not a spec failure, but causes "audio quality" rejects. If you record chapter 3 in a different spot than chapter 1, reviewers notice the room change. Keep a 10-second room tone sample from each session.
5. Edits creating micro-clips
Quick cuts at zero-crossings sound fine in the room. ACX's automated QC sometimes flags these as distortion. Use a short crossfade (2–5ms) on every edit point.
Run these checks before every upload and your reject rate will drop significantly.
What's the weirdest reject reason you've ever gotten?