Acoustic communication sounds evolve when receivers begin to attend to sounds that were previously an incidental byproduct of another process. Here, we review the physical acoustic mechanisms by which wings intrinsically produce sound. The first mechanism intrinsic to all flapping wings is the aerodynamic reaction to lift and drag, in the form of low frequency, tonal sound. This mechanism is responsible for hummingbird humming and the tonal wing whine of mosquitoes and other insects. The second mechanism is production and scattering of turbulence by the wing, which usually generates atonal sound. The third mechanism is friction: elements within a wing (i.e., feathers) slide against each other, generating localized vibrations that are transmitted to the air as sound that is broadband (atonal). This effect appears to be widespread in bird flight, and possibly in some large insects such as grasshoppers. The fourth mechanism is collisions (percussion), such as when a wing collides with another object, and generates impulsive, atonal sound. A few birds and possibly one bat produce communication sound in this way. The fifth mechanism is a stiff element that can snap back and forth between two stable states (a tymbal). Insect cuticle is predisposed to do this, and this usually atonal mechanism has evolved several times in insects such as grasshoppers and butterflies. The sixth mechanism is flutter: an element activated by airflow oscillates at a natural frequency. Feathers are predisposed to flutter and this mechanism is widespread in birds. We finish with examples of sounds for which the physical acoustic mechanism is not yet understood, including the drumming sound made by Ruffed Grouse, and clapping (such as of dove wings). These sounds might be the product of a seventh mechanism, trapped air escaping from a confined space, similar to the mechanism produced by clapping human hands.