The men talking on the surface of the Moon

Sound is a mechanical wave that requires a material medium, such as air, water, or solid, to propagate through the vibration of molecules [t2]. On the Moon, there is virtually no atmosphere, creating a near-vacuum environment [t2][t5]. Because sound waves cannot travel through a vacuum, they have no medium to carry the energy from the source to the listener's ear [t1][t3]. Consequently, astronauts on the lunar surface cannot hear each other speak directly through the air [t3]. To overcome this, they use wireless communication sets that utilize radio waves, which are electromagnetic and do not require a medium to travel [t3][t4]. While sound can travel through solid contact (like touching helmets), it cannot propagate through the lunar 'air' due to the absence of sufficient gas density [t2].

Sources

1. [1] https://www.science.gov/topicpages/l/lunar+dust+environment.html