Why can astronauts hear each others voices when their helmets are touching?

Astronauts can hear each other's voices when their helmets are touching because sound can be transmitted through solid materials, such as the helmet's surfaces. When one astronaut speaks, the vibrations of their voice travel through the helmet and into the other astronaut's helmet, allowing them to hear each other despite the vacuum of space surrounding them. This method of sound transmission is similar to how people can hear each other when they are connected by a solid object, like a wall.