Growing up around old Hollywood it seems to me that many posters have got this right. Elocution coaches (and classes in school ... my aunt taught elocution in high schools), the theater training of the day some of which was stylized some practical ... they had to be heard clearly in the back rows, and then there was the primitive microphones, amplifiers and optical sound tracks of the day ... all were slower in response and had a more limited frequency range than we do today. There still is the mystery of why almost every singer sounds like a southern Californian when they sing, no matter what their native accent. It's sort of like we are the final average of what the English language sounds like. Or maybe it has something to do with Hollywood!