The fact is that Americans, especially American women, are conditioned to speak more loudly than some other nationals. It's not a generalization, but a fact. It starts in high school, where you need to put an edge on your tone in order to be heard in the crowd. (This is a musician speaking.) Most Americans are not aware that they are doing it.
Anecdotal evidence on your part only forms an opinion for you. I've been to enough places outside the USA to throw water on your "fact".