Problem with sound volume

Hello, suppose you are playing a sound and I tell you that you should gradually lower the volume to the sound and stop the sound when nothing is heard anymore.

How will you know when you don't hear anything to stop the sound?

There is no function that indicates this, so how do you know?

since sound volume is displayed in db, just set it to stop once it reaches -100db. You aren't going to hear anything even before that, but it doesn't matter, it's a nice, safe round number