Sound is basically just molecules that are pushed closer together or pulled further apart from each other. If the pushing and pulling happens fast enough, the molecules form into waves that spread out, and we start to perceive this as sound. This can happen both in air, water and other materials, such as wood and metal. Sound travels at different speeds through different mediums. In air this happens at roughly 343 meters per second, in water 1481 m/s, in iron 5120 m/s.