Maybe we are saying the same thing differently. I might not be describing it right. The earliest flat speech that is spoken by regular born here Angelinos (that aren’t Kardashians) has no or few dipthongs in the vowels. Like the word “bag” has a very quickly pronounced, brief, “a” sound as in cat. The twang, like you say, probably did come from all the Midwesterners who migrated west, but if you hear the movie stars of the 1930s in interviews, they all have a little twang, where the word bag would be pronounced with a longer vowel, like “ba-yg.”
The first recorded evidence of Angelinos speaking flatly would be the classic announcer voice (ted Baxter style or gary Owens) used by broadcasters. Now we all talk like that here. (Unless coming from another country or state) (or Bakersfield! Still got some nice twang up there!)
Another example of a flattening of the American accent would be the Limbaugh brothers. Assuming they spoke the same way as kids. Rush went early into broadcasting and clearly flattened his vowels deliberately.
How many movie stars of the 1930s were from here? Hepburn was from Connecticut, Clark Gable was from Ohio, Cagney from New York City, and Gary Cooper was from Montana. Leslie Howard and Boris Karloff were British, Errol Flynn was Australian, and Bela Lugosi was Hungarian!
No doubt though that speech patterns have changed throughout the years here, especially since the 1960s. And we haven’t even started on the Valley Girl accent, Wiggers, stoners, and other variations! One thing that really bugged me about the old SNL skit “The Californians” a couple of years ago was that everyone used Valley Speak. The whole deal about the best freeway route was dead on but they really blew it by not including more variations in speech patterns.
Rush Limbaugh is of German ancestry from probably 19th century German settlers in Missouri and they rarely picked up the southernized accent. He has a little bit of it and can imitate it to perfection, though, as when he does Clinton.
Most Midwestern big-city accents are strongly influenced by German pronunciation. That’s mainly where the Germans immigrated to.
I was recently in the Arkansas Ozarks and heard a local town councilman speaking with the broadest accent I ever heard; he said ‘whar” for “where” and “thar” and “I was not awar of...” something. Then this other guy came along from the hills of North Carolina and guy # 2 spoke in exactly the same accent. You couldn’t have told them apart. So with that distance between them in terms of miles and area, I am assuming that that was the original way of speaking for what you might call Scots Irish hills people. It had just the slightest tinge of an English rural accent. I found it very interesting.