  The implications of the hidden, spontaneously broken symmetry for the
properties of the sound waves of a solid are analyzed. Although the discussion
does not go beyond standard wisdom, it presents some of the known results from
a different perspective. In particular, I argue that, as a consequence of the
hidden symmetry, the equations of motion for a sound wave necessarily contain
nonlinear terms, describing phonon-phonon scattering and emphasize the analogy
with the low energy theorems for pion-pion scattering.
