I've noticed, when I am watching old sitcoms and films from the 1940's, 50's, and early 60's, that the accents the characters use when the speak are...different. I apologize, it's really hard to explain how I percieve the difference. It's not really any particular regional accent, it's pretty generic when you come down to it, but it's so different from the way actors speak on TV/movies in the more recent decades. I also noticed that my grandma has this accent, especially when she is answering the phone or speaking in public. It's kind of a bright, refined, accent... they emphasize consonants and speak pretty clearly, they typically round out their mouths to make certain vowel sounds longer and more pronounced in words like "down" and "water" so they sound like "dowwwwwwn" and "waw-ter," and they take great care to pronounce the "H" in words like "What, why, where" so that they sound like "Hwat, Hwhere, Hwhy"... Think of how Julie Andrews, for example, talks in the Sound of Music. You can just tell it's an old movie even if you can only hear the audio track and not see it, by the way she and the kids talk.
Maybe it's just the result of formal voice/speech training that the actors did in those days, kind of like how newscasters have to learn to speak in a kind of non-accent... Does anyone know a thing about what I'm trying to describe here? I fail!
edit: i just realized Julie Andrews normally has an English (UK) accent, but in the Sound of Music, her elocution in speech and songs is pretty similar to the American ones I'm talking about, I just can't think of any other mainstream examples.