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Think of the world between, say, 1890 and 1950, and there's a good chance you'll picture one in black and white where people all talk using the Transatlantic Accent: a particular, high-pitched way of talking that made everyone sound like a gangster who spent a few years in a London boarding school. It wasn't really like that everywhere (in fact, there were many more regional accents than there are today), but it seems that way due to class and technology. How Stuff Works explains why the past sounds the silly way it does. 

Sources: How Stuff Works