We should note that ACX Check needs at least 3/4 second of “clean” background noise (room tone) to perform a good measurement. The inter-word noise in that test clip comes in somewhere in the -70s (normally good) but ACX Check reports -60.3dB. The limit is -60dB. The first test at ACX is The Robot which does an automatic test similar to ACX Check.
You should have a terrifically good idea the requirements of chapter leading and trailing room tone as well as all the other technical requirements. There have been several bounced performances from simply getting the number of seconds of silence after a chapter reading wrong. These errors are recursive because remember, you can’t “patch up” the submitted MP3. The MP3 compression quality goes down if you try. You should go back to your Master WAV archive to fix that kind of error, and then make a new MP3.
https://www.acx.com/help/acx-audio-submission-requirements/201456300
How to send a test (if you get that far).
https://forum.audacityteam.org/t/how-to-send-a-test-to-acx/49588/1
Two New User recommendations for microphone spacing, with and without a round blast filter.
Koz