"That tinny, clipped tone of yesteryear is called Transatlantic speech," said Jay O'Berski, assistant professor of the Practice of Theater Studies with Duke University.
Transatlantic is a specific style of speaking, or dialect, that is still taught in acting schools and was part of the curriculum for performers and broadcasters in the early days of mass media. "It's an effort to neutralize regional dialects and consciousness of a particular class," O'Berski said.
That high-end, nasally quality, also associated with speech from previous eras, is a very real phenomenon, although it may have more to do with technology than performance technique.
"My understanding of the high-end, all-treble sound is that it's a holdover from radio and the first talkies when they had very little bass technology in receivers," O'Berski said. "You literally could not hear bass tones before stereo technology."
So what we think of in popular culture as "that 1950s" voice is partially the result of performance techniques and technology of the day. The artifacts we have from that era are, to a large degree, recordings of professional performers and broadcasters – film and TV actors, news readers and sports announcers.