ACX rejected my BookyAI Audio files with "RMS is too low. Raise the level of this file X dB", although the app showed the RMS inside the range. What should I do?

Update BookyAI to version 1.9.13 or later, then on the Audio page click "Start fresh" and "Render" again (or use "Re-render" on the rejected chapters only). ACX measures the loudness (RMS) of the whole file, pauses included. Earlier versions of BookyAI Audio measured only the spoken parts, so a file could read -20 dB in the app and -24 dB for ACX once pauses and quiet sentence endings were counted. Since 1.9.13 every chapter is finished on the same measure ACX uses and is guaranteed to land between -23 and -18 dB, with a safety margin. The "RMS" chip next to each chapter now shows that whole-file value. Peak and noise floor were never affected. If you use the single complete file, click "Re-build single file" after the new render.