They form a background for other sounds and define a certain location, place or time.
Examples might be the sound of the sea (when you’re near the coast) or the sound of traffic in the modern city.
Keynote sounds are so common that they are often not heard. But they are always there and will influence the way in which we hear other sounds.
Keynote sounds are usually created by nature (geography and climate): wind, water, forests, birds, insects, animals. But can also be man made, for example: traffic has become the keynote sound in many cities.