One cause of this is sending 5.1 audio through two speakers, where the center channel just gets dropped along with a lot of dialog. Another cause is of course crappy mixing.

I have never seen software or receivers that drops the center channel instead of doing a proper downmix. The exception is when the user has incorrectly set up the speakers to identify as 5.1 when they have only a 2.0 or 2.1 setup.

I think it’s this, if you want to have the volume at a level where conversations are understandable, the action scenes or other “high tension” moments are way too loud, and you spend your evening ride the volume control…

Yea, this is our family too. Volume button up and down, up and down, for 3 hours. I don't recall this being a problem with older movies, either. I feel like maybe modern moviemakers have access to sound pipelines with crazy vast dynamic range, and audio engineers feel they have to use every setting from "mouse quiet" to "nuclear bomb" in order to do their jobs right.

The best thing I ever did for my den was put one of these between the TV and the speakers: https://www.fmraudio.com/rnc.html

Twiddle a few knobs and now everything is the same volume and I can keep it at a nice low level and not wake the kids, but still hear everything.

Interesting. Audacity has various compression settings. Have you tried that?

Sure, but my TV can't run Audacity :)

Admittedly I haven't tried this so probably misguided, but if you can stick that box in front of the TV, could you not put a computer instead?

Yes, a computer with a good low-noise ad/da converter can be an excellent compressor. But it's likely to be much more expensive than a used RNC.