They're alive and start singing.
They vibrate on various areas (surface, strings, etc.)
They use echolocation to make a sound wave.
They use the Doppler effect to make sound.
The Doppler effect was discovered by Albert Einstein.
Bats and dolphins use echolocation to find their food.
Quality makes sound louder.
When somebody bakes a cake.
When the sound can effect different types of buildings and knock them down.
When the roar of a siren is louder coming toward you then going away from you.