No doubt you are correct but as a Ham, you might appreciate the variance in the ability to deal with sound. Some ops, can learn 20wpm fairly easy. Many years ago, if I'd known how hard it was to get to 20wpm, I doubt I'd taken the task on. I made it... but only because of being too stubborn to give up... it was really hard. It required a year of work with almost every evening devoted to an hour of practice.

As you say, if I changed the mixtures and flew then I might be able to sort out what sounds were not prop load related or positioning of the exhaust to my ear... and if that was what was needed to ensure good operation, I'd likely put the effort to it. Fortunately, I don't think it is necessary as I believe I can do about the same on the ground without the load changes and positioning changes so that my ears don't have to sort out those subtleties from the telltale sound of an engine wanting a richer mixture.

I completely believe that some can do so... but I also believe that some can't.