That's a really good explanation, thanks for sharing.
I've thought of an another factor, which he didn't mention, I don't think, and it's a visual factor, well, two really.
The video explanation referred to how back in the old days, there would be a big microphone and the actors would speak towards them, whereas nowadays there are probably a couple of booms and the actors with dialogue in the scene are probably mic'd up. And there was mention of more naturalistic voices nowadays too.
I think in older movies and television, perhaps actors were more likely to have a background in theatre and would be more likely to (a) project a bit more and clearly enunciate as if they were in a theatre production, and also (b) face the audience. I think there's an element of lip-reading that goes on, even with people who aren't deaf/Deaf or don't have any level of hearing impairment. I think we all lip-read a bit, to some extent. And so if the character has their back towards us, it makes it harder for us to 'hear' what they're saying.
And as well as different staging, with actors facing away from the camera more in modern day film and television (rather than a theatre-type stage setting with characters facing the audience (camera)), there's also the fact that many more scenes nowadays are dark, with minimal lighting, so you can't see the characters as well, and that also gives rise to difficulties doing the kind of unconscious lip-reading that adds a bit of extra non-verbal communication, helps us fill in the blanks between what we hear and what we see the lips saying.
Digital imaging technician Nicholas Kay and 'Euphoria' cinematographer Marcell Rév explain why films and TV shows might seem visually dark.
variety.com