I'm not sure the actors are mumbling their words. I think the issue stems from the way sound is decoded on people's devices. Movies used to be mixed down to stereo and TVs have no problem producing the sound. Now that it's possible to stream, say 7.2, the TV will tell the service to send something like that but the TV does a poor job of outputting the center vocal channel through it's two rear $3 speakers so you can hear it. The same audio track decoded with a nice cinema amp and two center speakers will generally be really clear. (Also some TVs have a setting to boost the vocals to try and fix this)
Again, with the dark movies. They seem to be color graded and encoded for HDR screens, and most HDR screens are really a fake type of HDR and so the movies just come out super dark. They look a lot less horrible on a really great high-end TV in a totally dark room.
That and actors that mumble their words...