Secret i learned on my raspberry pi running stereo speakers on Kodi is you can set a seperate volume for the dialogue channel so i just bumped it up like 14 decibels and now it matches the action fairly well. You can set it from the audio settings inside the movie and its called something like center channel downmix i cant remember exactly
I noticed this as well a few months ago and it did change the monitor brightness for real, but now it broke on my system and wont do it anymore