Its amazing how slow human beings are. The atoms inside your eyelash collide with one another a million million times during each time you blink your eye. Typically only vast conspiracies of immense numbers of atoms can produce the slow behavior that humans can perceive.
Crystals are rigid because of the broken translational symmetry.
Because they are rigid, they fight displacements. Because there is an underlying translational symmetry, a uniform displacement costs no energy. A nearly uniform displacement, thus, will cost little energy, and thus will have a low frequency. These low-frequency elementary excitations are the sound waves in crystals. A good example is given by sound waves.