When the molecules in the air become less dense (as in the process of hearing), has occurred.

Explanation:

Whenever there occurs a decrease in the density of a substance then this is known as rarefaction.

For example, when particles of a gas become more dispersed or spread out then it means they have gained more kinetic energy. As a result, there will occur a decrease in mass of the gas and increase in volume of the gas.

As density is mass divided by volume. Hence, decrease in mass will also lead to a decrease in density of the gas.

Therefore, we can conclude that when the molecules in the air become less dense (as in the process of hearing), rarefaction has occurred.