Full disclosure: I've read the book, about a thousand years ago. I haven't watched any of it, and don't intend to.
Mumbling. Currently, there are a few things in TV-land that may be relevant.
1. People who don't enunciate adequately (there's a list...). It's perfectly possibly to have a non-rp voice which enunciates correctly. I was trained to project and enunciate, and it happens automatically if I raise my voice, even more than 30 years later. It's a technique thing, so either they don't teach it any more, or people don't bother.
2. Dubbing mixers who listen on their fuck-off expensive surround sound kit and don't check audio on their cheapie-cheap near-field monitors*, which leads on to
3. R128, whereby the EBU and others have decided that it's OK to expand the dynamic range of television shows, as long as the overall loudness remains within certain limits over the duration of the show. The dynamic range of TV used to be fairly constrained, and vox and FX were ridden as required to keep the vox at a fairly consistent level. The advent of R128 means that you now have to ride the volume control at home, and you end up with the movie problem where you get the vox to the right level just in time for the FX to break your windows.
*Assuming you can find a mixer who can actually mix, and actually listens, rather than someone who does it all at home on the meters and the plugins on their hooky copy of Pro-Tools, and then emails the file.**
** Jaded and cynical? you betcha.