I've seen a bunch of complaints about dialogs being muddled, here at ASR, by people with good speakers one would assume.
It's hit and miss in my experience. For example, the relatively recent (great) movie Arrival has clear dialog, even at some points when some character does mumble. Nolan ... well anyway, he also famously uses film stock and refuses digital, and has his quirks for sure.
What I wonder is if now that many movies are getting produced by Apple, Netflix, Amazon Prime etc, if the target delivery system will be a 5.1 or 7.1, instead of that ridiculous 128 channel idea...