When birds stop singing something changed in the environment. We unconsciously pick up on this.
It is how we pick up that what was once safe, is no more. It is why birdsong in a garden is such a deeply soothing experience.
We still have this ability, and need, to listen out for danger cues: When the moment hums at us, everything is humming along nicely we know all is well.
When the humming stops, what went away? What needs to be added to start humming again?