Hi Wolforgan,
In the post that you quoted, I simply said that if you’re having to send the phone back because of an unsolved problem, it’s always worth performing a factory reset before you send it. You should do it anyway, on principle.
However bear in mind that a “factory reset” is not the same as re-installing the OS. It just deletes all your settings, data and the apps you’ve installed. So the expression “factory reset” is not very accurate, it’s somewhat misleading. It doesn’t restore the phone to “the state in which it left the factory”, despite what hundreds of web articles on the subject may say.
How likely is a FR to be a solution? It should solve problems that come from settings, apps and data that you have put on the phone. It will not solve problems caused by errors in the operating system or default settings.
As to the original problem (call volume too loud with and without loudspeakers) we’re talking different FP models and therefore Android builds so what might have “worked” for those on FP3 a year ago is not necessarily a solution for FP4 today. Unfortunately in the other thread, people didn’t get back to post the solutions that worked for them 
If you are running the latest version of stock FPOS on your FP4, I would expect a “factory reset” to solve your volume problem. I wouldn’t be surprised to find that it came from one of the communications apps like Whatsapp, Signal or whatever, but that is pure speculation.