General equations for acoustic‐gravity waves in a fluid are derived as a particular case of the theory of elasticity of initially stressed continua. The differential equations for the fluid dynamics and the corresponding variational principles are obtained from the more general results for the elastic solid established earlier by the writer. The transition from solid to fluid is illustrated for the special case of a constant gravity field. The dynamics of a fluid under initial stress is thus brought within the scope of the theory of elasticity providing a unified treatment of wave propagation in composite fluid‐solid systems.