Specific to the early decades of radio in North America, a good example might be the Mid-Atlantic Accent or Canadian Dainty, both falling out of favor around th

Specific to the early decades of radio in North America, a good example might be the Mid-Atlantic Accent or Canadian Dainty, both falling out of favor around the early 1950's, about the same time the last nail was hammered into the radio drama coffin, by my listening. These are accents though (not dialects) and consciously learned ones at that, but widely used during their short existence, then fizzled out quickly as broadcast media became more widespread.