A theoretical model of harmonic perturbations in a compressible turbulent mixing layer is proposed. The model utilizes the triple decomposition method. It is assumed that the instantaneous velocities, temperature, and pressure consist of three distinctive components: mean (time-averaged), coherent (phase-averaged), and random (turbulent) motion. The interaction between incoherent turbulent fluctuations and large-scale coherent disturbances is incorporated by the Newtonian eddy viscosity model. The governing equations for the coherent disturbances have the same form as in laminar flow with substitution of the Reynolds number and the Prandtl number by their turbulent counterparts. A slight divergence of the flow is also taken into account. Theoretical results and comparison with experimental data reveal the significance of interaction between the coherent and random constituents of the flow. The model is applied to the case of a mixing layer over a cavity in order to predict the resonance frequencies.