Question about Samsung QE55QN90C

Yes, this is also the case with my Samsung TV. I was able to change this in the "smart mode settings".
There are different settings to change this, you have to try a bit to keep the sound pretty much the same volume.
Press the home button on your TV remote control:
- then press the left arrow button, (this will take you to the list menu).
- navigate to "Settings" with the lower arrow button
- then use the right arrow key to navigate to "All settings"
- Continue to "General and data protection"
- then go to "Smart mode settings"
- then deactivate "Adaptive sound+"
- and deactivate "Adaptive volume" possibly also deactivate "Active voice amplifier" (does not necessarily have to be, try it out as required).
P.s.As "schweizyb" also mentioned, you can go to "Expert settings" under "Sound" and activate the "Automatic volume setting" menu.
Have fun trying it out

What do you mean by this?
If I'm right, you can go to Audio and Expert settings and set Maintain same volume so that it is constantly the same.

LG Leo (: