common complaint about movie audio is ‘the speech is too quiet and the action is too loud’. this can be resolved with dynamic range compression or normalization. ffmpeg provides a normalization filter called dynaudnorm which u can use in mpv via --af=“dynaudnorm”.
This was about to be my response. Sometimes, when a 5.1 surround mix is forced into stereo output, it causes the ‘speech’ channel to sound quieter than it should.
Try sticking to non-surrounds sources OP if you don’t have surround setup