A hybrid imaging system capable of combining the spatial resolution of ultrasound with functional sensitivity of fluorescence optical imaging can render the conventionally ill-posed optical image reconstruction more tractable. In this paper, ultrasonic modulation of diffuse photons from an incoherent source in a turbid medium or FluoroSound has been proposed as a mechanism for dual-modality fusion. Theoretical calculations based on a diffusion approximation for tissue depths up to 2cm and reduced scattering coefficients ranging from 5-20/cm have shown show that diffuse photon modulation increases with scattering, decreases with absorption, reaches a minimum when the acoustic focus is located mid-way between the source and the detector; and can be optimized by a suitably shaped and sized acoustic focus. The diffuse photon modulation signature could potentially be used for improving spatial resolution of deep tissue fluorescence imaging and enable fusion of ultrasound and optical imaging in a single measurement.