The accents would have been considered neutral (mass media) American accents of the period. The two men have a educated northeastern accent. Later, the Western/California accent became the mass media accent. I assume that, somewhat like news announcers, the actors were trained to be very clear in their diction. Very clear diction may have been a way to compensate for the poor sound systems in theaters at the time.

Fun question, Mike. I’m an American English native speaker and I have to say that Marylin’s accent is sort of unusual. But there were a couple of American actresses who had a similar way of speaking, including Shirley Temple. I believe, as Simon said, many of them may have had some special lessons and even corrective therapies to speak clearly for films and singing. But in general, all the accents in this portion of the video are neutral American accents. I have always noticed that the speech in many American films from the 1940s to 1960s was clearer than it is now. I hope this helps.

I am from the UK so I am not an authority of american accents but apparently Marilyn Munroe had a stutter and a speech therapist trained her to speak in that strange breathy way to overcome her stutter. https://www.vogue.com/article/marilyn-monroe-five-things-you-didnt-know

Fun question, Mike. I’m an American English native speaker and I have to say that Marylin’s accent is sort of unusual. But there were a couple of American actresses who had a similar way of speaking, including Shirley Temple. I believe, as Simon said, many of them may have had some special lessons and even corrective therapies to speak clearly for films and singing. But in general, all the accents in this portion of the video are neutral American accents. I have always noticed that the speech in many American films from the 1940s to 1960s was clearer than it is now. I hope this helps.