Then I noticed that native speakers also complain.
Then I started to watch YouTube channels, live TV and old movies, and I found out I could understand almost everything! (depending on the dialect)
When even native speakers can't properly enjoy modern movies and TV shows, you know that something is very wrong...
It only works if you're watching in a room that's acoustically quiet, like a professional recording studio. Once your heater / air conditioner or other appliance turns on, it drowns out everything but the loudest parts of the mix.
Otherwise, the problem is that you probably don't want to listen to ear-splitting gunshots and explosions, then turn it down to a normal volume, only to make the dialog and whispers unintelligible. I hit this problem a lot watching TV after the kids go to bed.
As much as I enjoy deafeningly bright explosions in the movie theater, it's almost never appropriate in the casual living room.
I recently bought a new TV, Bravia 8ii, which was supposedly not bright enough according to reviewers. In it's professional setting, it's way to bright at night, and being an OLED watching HDR content the difference between the brightest and darkest is simply too much, and there seems to be no way to turn it down without compromising the whole brightness curve.
But also, people in old movies often enunciated very clearly as a stylistic choice. The Transatlantic accent—sounds a bit unnatural but you can follow the plot.
In Poland our original productions have so badly mixed sound that in almost none series in my native language I cannot understand without captions.
But the upside of it is - with English being my second language - I understand most of movies/series I watched.