I notice the same thing. Its one of the things I enjoy about watching old flicks, the older the better. Too bad sound didn't come in any earlier.
People sounded educated back then. The “class disparity” wasn’t between those who had money and those who didn’t, but between those who were educated and those who were not. There was always a vast difference between the poor shop girl who spoke perfect English and the rich wife of a mobster who spoke using poor grammar and slang.