Sorry, but I work with a lot of Hollywood production companies and have been on plenty of sets, and this is a load of crap.

Today's actors and actresses speak their lines just fine. The problem is the trend toward louder audio f/x and music, which can drown out voices if not mixed correctly. See, e.g., Tenet for a horrific example of audio mixing, and the characters can barely be heard even when yelling; compare to The Batman where Pattinson rarely speaks above a whisper but is audible and understandable in every scene.