I was listening the other day to a program where Claudette Colbert, Paulette Goddard, Jeanne Crain, and Jinx Falkenburg were Jack's dates to the premiere of his movie. What struck me was how affected all of these actresses sounded. They all sounded like they had graduated from fancy prep schools and were in their 50s or 60s (instead of their 20s and 30s). What a contrast with Mary's very natural speaking style. I've thought the same thing when other actresses appeared such as Barbara Stanwyck. Of course, it's not just limited to actresses. Ronald Colman, Herbert Marshall, and other actors also had an overly affected way of speaking (at least by today's standards). But while this is true of some actors, it was true of ALL of the actresses who guested on the show.
Today, this sort of finishing school diction has gone out the window and, for better or worse, today's actresses sound pretty much like everyone else when they're interviewed on The Tonight Show or David Letterman.
One of the reasons Jack Benny's program works for modern audiences is because the cast is talking in a natural informal way. But everytime a 1940s Hollywood actress appeared, the illusion is temporaily lifted and we're reminded that we're listening to a recording from a different era.