A macroscopic system in thermodynamic equilibrium is in its most probable state, i.e., the extremum of the corresponding thermodynamic potential. Because potentials are generally quadratic in the vicinity of extrema, small deviations of the volume from equilibrium will be met with a restitutive force that is linear in the deviation, i.e., harmonic. If the damping is not too strong, such a deviation is relaxed by oscillations, which we call sound. More generally, any harmonic excitation of the equilibrium state can be called sound. Thus, in magnetic materials there are spin waves, oscillations of local magnetization, and in superfluid helium there are elastic waves of temperature, so-called second sound. Due to its most general possible thermodynamic origin, sound is an unavoidable mode of interaction between macroscopic systems, and research into sound encompasses broad areas of physics, technology, biology, medicine, sociology, and art.