On many smart TVs or Windows computers, there is a feature called smart audio adjustment. It basically prevents the audio from going too loud or too quite when playing files that has audio output, such as a movie. I like to know if there is a similar feature or if there is an OSX application that can accomplish similar goal.
Boom3D (paid, $17) can do this system wide for all applications. Enable "Night Mode" in settings. The little slider (headphone icon on left, speaker on right) determines how aggressively the program will limit volume changes.
(I highly encourage disabling and ignoring all of this program's other "features", except for 3D Surround when applicable. The voodoo processing will ultimately just distort whatever you're listening too.)
The technical term for this is "Dynamic Range Compression", although you won't see Boom3D using that name either.
Edit: I didn't realize Boom now cost $17—I'm pretty sure it was closer to $10 when I first bought it. The Dynamic Range Compression does work well, and I'm not aware of any alternative programs with the feature, but that price is a little steep.
That depends what app are you using.
For example iTunes, you can set that up in preferences.
For example VLC player, you can set that up in Audio Effects.
It all depends what app are you using.