You are correct that sound waves require a medium to propagate, and since Mercury has no atmosphere, sound waves would not be able to travel through it. Therefore, sound as we typically understand it would be absent on the surface of Mercury.
However, it's worth noting that the term "sound" can be used more broadly to refer to any mechanical vibration or wave that travels through a material medium. In the case of planets like Mercury, seismic activity can generate vibrations known as planetary or seismic waves.
Mercury experiences seismic activity in the form of internal movements and quakes caused by various factors, such as the cooling and contraction of the planet's interior, impacts from meteorites, and gravitational interactions with other celestial bodies. These seismic waves propagate through the solid material of the planet itself.
While we wouldn't be able to perceive these seismic waves as audible sounds, scientists can use seismometers placed on the surface of a planet or in orbit to detect and study these vibrations. By analyzing the characteristics of these waves, scientists can gain insights into the internal structure, composition, and dynamics of a planet.
So, while the absence of an atmosphere on Mercury means that sound waves as we typically think of them would not be present, there are still seismic waves generated within the planet that can provide valuable information about its interior.