Question
Is it possible to enable the lower audio track only when there is no sound in the upper audio track when there is an audio track as follows?
I understand that there is a way to cut the lower audio track and put it in the gap, but it takes time and effort, and it is difficult to adjust the volume all at once after inserting the gap.

Answer
This is possible with an editing method called audio layering.
This is possible on the Edit page and the Fairlight page.
procedure
Select [View]-[Show Audio Track Layer] and check it.
Layers will be available within each track as follows:

Select all audio to overlay (overwrite)

Hold down the Shift key and drag down
By pressing the Shift key, you can drag without shifting the time.

Can be layered within one track.

Now you can mask (disable) the lower audio with the upper audio in the same track, and the orange part in the following image will be played as audio. (The image is colored for illustrative purposes, but it will not appear this way in DaVinci Resolve.)

#layer