After doing dome hardware tests and analyzing files in audio editors, it is obvious to me, that the poor sound experience that I have with some video files is due to my operating system mixing multiple tracks into just two with awful merging result
Long story short: 5.1 / 7.1 audio tracks attached to video files look absolutely normal in audio editors - but when played and recorded with the record-what-you-hear method they show later on extreme clipping on the waveform. And this is my stunning discovery (thus concussion) after first having done some extended auditory test and thinking, that it had to do something with the too new / sophisticated codecs and not adequate / old hardware for them. But having an old external 5.1 sound card, new integrated 7.1 sound card, old wired stereo headphones, new wireless stereo headphones used with wire and with USB Bluetooth transmitter, old stereo system with loudspeakers - I took time and tested various (stereo) hardware configurations (i.e. methods of connections). And conclusion was always the same: 7.1 is garbage or sometimes does not even play at all (showing just a single video frame in the player) while some 5.1 tracks are bad but sometimes / somewhat acceptable. But as I said the what-you-hear shows me very clearly that I am not imagining things and there is a profound difference between 7.1 and 5.1 and that also 5.1 as standalone samples look bad (like extreme cases of music destroyed with loudness war)
And so the question is: what is happening here?
I am running Windows 10 Enterprise 20H2 19042.746 on Gigabyte X570 Aorus Ultra rev. 1.2 motherboard. I never had problem with stereo audio tracks while this with 5.1 / 7.1 I have noticed only recently after buying those new stereo headphones. At first I thought that it was these new piece of hardware, that it had problem with playing some frequencies- but then I also checked my other hardware and the issue was still present (but just heard in a much less profound way as those headphones have great noise isolation and so on)