What you're describing is called dynamic range compression, and mpv can be configured to do this in multiple ways.
Back in the physical media days, it was pretty common for DVD/Bluray players to include this feature. Unfortunately it's not something that streaming app developers thought twice about. Your TV or streaming box also may or may not have the feature.
It's a problem for me between being half deaf anyway and Google not adjusting source streams to any sort of minimum decibel level. So listening to the local Pacifica station with no adjustment is about half the loudness of the local NPR station. Pleas to correct the audio level have fallen (heh) on deaf ears, so my donations and my listening has moved elsewhere. Sorry, local radio. I'd support you if you'd support me.
Doesn't that depends in the sound system setup ie hardware? I tried vehemently to fix it once with JamesDSP with no avail. My understanding then was the need for hardware support like Dolby or something. It was always an issue with music and dialog.
It's interesting that even podcasts, which you'd think would be more focused on audio considerations, don't do this, either. I'm mostly thinking of Dan Carlin's podcasts, where sometimes he goes a weee bit quiet. (And Shadows of Utopia, which I have to jack up the volume for the entirety of)
Back in the physical media days, it was pretty common for DVD/Bluray players to include this feature. Unfortunately it's not something that streaming app developers thought twice about. Your TV or streaming box also may or may not have the feature.