The waves of sound are actually nothing but contracting and expanding air! Air contracts when it is somehow put pressure on. For example, when the musician playing Guitar tinkles the strings of the instrument the reversing cord pushes away the air in that part. In fact, the air contracts rigorously. A moment later it expands again, and thus, to make space for it the other air contracts! And so it keeps passing further on! The process of expansion and contraction slows down in regular succession, because the force generated at the start reduces gradually. But in the meanwhile if one contraction touches one’s ear and the same air then expands it would naturally stir the eardrum! Its signal outright reaches the brain and the brain would identify it as sound.