Language Mystery: When Did Americans Stop Sounding This Way?

The Atlantic's wonderful new Video Channel has a lot of great material, and it is invidious to point to any one offering rather than another. But this clip, "Wings Over the Golden Gate," which I started watching because of the irresistible aviation + California combo (I have flown over exactly the scenes shown here) got my attention for another reason. Watch for about 60 seconds and you'll see what I mean.

The language that the narrator, one Gayne Whitman, uses is florid enough. But his accent! It's instantly familiar to anyone who's seen old movies and newsreels from the 1930s and 1940s. But you cannot imagine a present-day American using it with a straight face. It's not faux-British, but it's a particular kind of lah-dee-dah American diction that at one time was very familiar and now has vanished. Margaret Dumont, in the Marx Brothers movies, was maybe the most familiar and caricatured female equivalent. Even Katharine Hepburn's very arch accent (eg in Philadelphia Story) seemed a step closer to "modern" American usage.

I wonder who the last person was who sounded this way. I wish someone still did. Maybe I'll try.
Mystery solved, or much of it. I knew the accent but must have been the last person not to know its name. I had always thought that a "Transatlantic accent" was the faux-British frumpery I mention above. Unt-uh!  As reader P. Hoh says in a representative response:

The accent you are wondering about is the Transatlantic accent, also called the Midatlantic accent.

This was not a regional accent. Rather, it's an accent that was taught to actors and announcers.

I learned about this accent from Amy Walker's "21 Accents" video on YouTube.
She starts using the Transatlantic accent at the 2:12 mark.

Glad I asked. But it still leaves the question of why it so totally fell out of fashion, and so fast. It's very hard for laymen to have a sense of how quickly accents changed in the past. Because there weren't movies and only a few recordings, we lack the vivid sense of how people actually sounded in the 19th century and before. But the total disappearance of what had been a prominent part of American public culture -- the formal lah-dee-dah tone -- in just a few decades is remarkable and must mean ... something.