When you shout in a hollow room the sound travels away from the source(mouth) there by disturbing particles in the air and hits the wall and again reflects back. This is how sound echoes.Sound travel with 350m/s in the air. Even when the sound is vibrating away still you can hear the sound a distant away.
Its nature’s gift for having all the gas particles, dusts, matter and atoms for vibrating. If not we can’t hear sound.