If we assume they’re using the same audiobook settings as ACX…
https://forum.audacityteam.org/t/measure-between-23db-and-18db-rms/32770/16
Audacity has an add-on tool that can automatically display all three (and more) of the critical sound readings—including RMS. This can be insanely handy because it lets you manage the sound quality at home without ping-pong shipping files around.
This should still be current:
http://wiki.audacityteam.org/wiki/Nyquist_Analyze_Plug-ins#ACX_Check
http://wiki.audacityteam.org/wiki/Download_Nyquist_Plug-ins#download
There are ways to measure the sound quality without this tool, but it’s a page of click this and click that instructions. The measurement tool is much better.
Let us know where you get stuck.
Koz