As the title says - When you watch American television shows from the 40s, 50s, even 60s, everybody had a certain accent that you don't hear today ever. It's particularly obvious when you watch clips of old news programs. But nobody talks like this today. When did this change and how?
Here's an answer by u/Dont_Do_Drama to a similar question asked a few months ago that might help https://www.reddit.com/r/AskHistorians/comments/bwf6ty/why_do_actors_talk_so_oddly_in_american_1950s/