Starting from macroscopic hydrodynamic and thermodynamic equations, the authors use linear response theory to derive Green functions for an isotropic solid in which thermal and longitudinal acoustic modes are coupled. Either density and entropy or density and temperature can be taken as independent variables, and Green functions are given for both cases. The fluctuation spectra are derived by means of the classical fluctuation-dissipation theorem, and it is shown that the integrated spectra have the form independently derived from thermodynamic fluctuation theory. The cross section for inelastic light scattering from a bulk specimen is derived.