What does Fair Loudness do?
It plays two versions at a matched level, so the louder one does not win the comparison on volume alone.
Turn it on and Soneam plays the versions you are comparing at a matched level, using each version's measured loudness. The client presses it, not you, which is the point: the bias it corrects is theirs.
Louder reliably sounds better in a quick comparison, whatever else is true about it. That is not a fault in your client; it is how hearing works, and it is why an A/B against an unmatched reference tells you almost nothing. Matching the level does not make the decision for anyone. It just removes the one variable that would have decided it by accident.
It only ever turns down. Matching attenuates the louder version rather than boosting the quieter one, so nothing is ever pushed into clipping and the file itself is untouched. There is a free version of this tool you can use on your own files without an account.
Not covered here? support@soneam.com.