Constant Gain Differential

can a process like it instead be applied to every sample of that track?

Sure. Start the process now. Come back next week.

The reason it’s called “RMS” is the electrical association. Root Mean Square is an electrical measurement that calculates the area under the wave—the blue waves—like on the timeline. By definition, you have to have waves and waves are large collections of samples. I can’t think of a good stupid story but it’s like trying to derive the speed of a car by looking at the car very briefly in one place as it goes by. Not enough information. If you look multiple times, you can easily use that data to calculate the speed.

The reason we’re talking about this at all is that RMS turns out to be remarkably close to loudness in audio. It’s handy and they didn’t have to invent a whole new measurement.

So. What I would recommend missing any other ideas, is you create two carefully processed shows by using the AudioBook Mastering suite.

The top part of that instruction page is really all you need. All the rest of it is what to do if something goes wrong while you’re reading an inch-thick novel.

That should give you two carefully controlled, almost identical tracks. Then, use plain simple Effect > Amplify and reduce the background track by the amount you wish.

Select one track by clicking just above MUTE. Effect > Amplify: Amplification -20dB > Enter.