This is similar to a question that I asked my father 15 years ago. (I was born in the early '50s, he in the mid '20s.) "The speech of Lowell Thomas on recordings sounds very different from our present day speech. Have things changed that much, and did people really speak as he did?"
My father's answer was that indeed people did speak (or try to speak) in the same way as Lowell Thomas did at important times, and that Mr. Thomas's speaking was never made fun of because it was so "normative".
Indeed, the "mid-Atlantic" accent was encouraged in all mass media in the U.S., because it was a true compromise of U.S. accents: the New Englander found it just as understandable and believable (and attainable) as the Californian or Okie (listen to Will Rogers).
It is important to put this all in a context.
From the time of the invention of the phonograph through the time that the children of television had grown up, there had been a movement in the United States for increasing the education (and common sense) of the people.
By about 1970, most people from all regions of the U.S. had been influenced (often subliminally) to modify their speech toward a "norm". Educators from all levels of society had pushed the norm with zeal. Booker T. Washington, Norman Thomas, Thomas Dewey, Walter Cronkite and Richard Nixon all spoke in public with a similar accent in spite of their very different upbringing and position.
As the broadcast media gained more and more penetration into the daily hearing of people, so did the art of rhetoric and formal oratory decay... if a pretty or handsome face brought more advertising dollars than a clarion voice, the choice was clear.
Concurrently, there were vast upheavals going on socially. The mid-Atlantic accent was often viewed as the voice of the rich and the white. (After all, who had the power in the country?) There was a strong movement, starting in the late '60s, to "democratize" many things, including speech.
It is no coincidence that the man of the 70s considered Richard Nixon's "expletive deleted"s to be important to be printed in their full glory, when 30 years earlier the same words of Gen. McAuliffe were edited by every American newspaper to read "Nuts!"
So yes, speech has changed. Especially public speech... which always influences the speech of the man on the street. And yes, the speech of the '50s, at all of its levels (colloquial, informal, and formal) was different from that of our day.
Need an example from history? Just look at the Great Vowel Shift in English, where a massive change in pronunciation (vowel lengthening and diphthongization) took place for about 100 years before 1450, followed for the next 250 or so years with more changes.