Subtitles becoming more popular, why so
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As a result the sounds were never really sent in one direction or another, the bass wasn't completely sent to the subwoofer, etc.
For the case of the oblivious majority (sadly I suspect there are more people who play surround on two channels than have a surround system) that means they can hear the dialog without a center channel, hear some bass without a subwoofer, etc. It means though that the people who bought a surround system and bought a disk with a 5.1 surround track didn't really get what they were paying for.
By the time Blu-Ray came out they got more aggressive on mastering surround which means you get a much better experience with a surround system but if you play that track on a two-channel system you'll barely hear the dialog at best.
I'm sure I've said it here before but I would love, love, love if movies/TV allowed sound control similar to video games: a dialog slider, a background music slider, a sound effects slider, etc. I don't understand why Netflix (or any other streaming platform, really) doesn't make this a priority for their original programming.
Agree, it's a feature I really appreciate, especially when an off-screen character says something crucial to the plot but hard to hear. I also get to read lyrics to songs on the soundtrack that I would otherwise have no idea of.
[1] The funniest show I've ever watched is Gintama in Japanese with subs, after all.
To fix that I usually preprocess them with subclean[1], i.e. extract the subtitles with `ffmpeg -i IN out.srt` in clean and use that srt file. There are some players with cleanup options, but I never found them comprehensive enough.
Also, separate subtitle tracks would be awesome, or a slider for "how much information" you want displayed.
There's another (but vaguely related) trick for getting the rear Left and Right channels, and then add a simple low bypass filter for full Dolby 5.1.
If you don't have a center channel in your stereo, but you do have a setting for Stereo Widening, there is another technique: Reverse the phase of one of your speakers by simply plugging in the + and - to their opposites, and then clicking on Expand or Widen or whatever your kit calls it. The middle / mono content will still be there, but only barely.
They're also good when you have children around where ambient noise might take you away from the dialogue.
I kind of dislike it though, I've grown to use it as a crutch and would prefer to not need to use it.
This is almost the answer.
The answer is, yes, naturalism, but it's not about difficulty recording.
TV and movies used to emphasize diction and clarity a great deal, where every word could be perfectly understood. The problem was that this could all feel a great deal "actory" -- it felt like you were watching actors on a stage performing for an audience rather than real life happening in front of you. Of course, for multi-cam sitcoms, they were quite literally actors on a stage performing in front of a studio audience.
Over the past 20 years, audiences have grown tired of how "fake" it seems. They've demanded more naturalism. "Stagey" multi-cam sitcoms have almost disappeared, almost everything is single-cam now. And with the greater "realism" of single-cam, speech has become much more naturalistic.
In real life, people mutter, muddle their consonants, speak low, etc. And so the same thing is happening on TV and film now, because it seems more true to life. The same way darker environments are more true-to-life, lighting has been trending more naturalistic, accents and ethnic casting are more accurate... on the whole, everything is trending towards greater naturalism. And so this includes diction.
So the problem in general has nothing to do with sound recording or mixing -- the technology behind these is better than ever. To the contrary, it has to do with a conscious artistic choice to prioritize naturalistic speaking over "actory" exaggerated diction. Realism over clarity.
And I suspect that it's actually been partly enabled by the fact that subtitles are so easy to turn on now, that for people who don't like this, they can just turn the subtitles on. That wasn't the case most of the time 30 years ago. (The same way you can turn up the brightness on dark scenes, if you must.)
My setup is wildly unmatched in that I have a cheap Yamaha RX-V683BL AV receiver (which needs to be replaced, but high cost of HDMI 2.1 makes this hard) and a 5.0 arrangement with two KEF LS50's serving as L/R and one more LS50 serving as the center. Now certainly the receiver and speakers should be paired better. However, even if I'm not driving them correctly my setup is magnitudes better quality than the average soundbar I see most people buying up.
Most of the stuff I'm watching is Blu-ray so the quality of the source isn't to blame here. So the question remains, is my equipment that poorly matched, is the mixing terrible, or do I just have hearing loss in the vocal frequency range?
In short, subtitles are for when sound is avavilable but hard to understand, while captioning is when sound is not available (such as the viewer is completely deaf). This is the definition from HTML5.[0]
Of course, one could simply mute the TV showing subtitles and then I guess they'd become captions!