The 707 would have been quite noisy but there are times when production requirements have to overide reality in terms of lighting and sound effects.
Yes, how many times have you seen people on TV driving down the freeway in a convertible and still talking in normal conversational voices? It's a convention of the medium, I guess. You don't really want to watch passengers raising their voices to be heard over the jet noise, and dialogue with a lot of "What?" in it.
Same thing with conversations in crowded bars and restaurants. Even if the bar is standing room only and music is blasting the main characters can always converse in a normal tone of voice.