America is a nation of loud people because it is a nation of immigrants.
It is noticeable that when English speaking people visit the rest of the world, they don’t make any attempt to learn their language – instead they talk to people in (for example) France using English. When the French do not seem to understand, the English simply repeat the same thing – louder and slower.
It is likely that newcomers to America were treated in much the same way. The original settlers were mainly English speaking (The Mayflower and all of that) and so when other people came along to America, the English speaking people of the established colonies would have attempted to teach them English.
When the immigrants (naturally) did not understand – the incumbent English speakers simply repeated the same thing – louder and slower.
Thus the new Americans came to understand that the way to speak the language of their adopted nation was LOUD and ssslooowww.
Hence America is a nation of loud people who speak with a ‘drawl’.