The polarization changes and symmetry properties of linearly polarized light passing through an isotropic medium with acoustically induced anisotropy are described for normal light incidence and for sound frequencies which are typical for ultrasonic light diffraction in the Raman-Nath and intermediate region. In the former case, a rotation of the polarization plane occurs in all orders of diffraction with the angle of the rotation being a function of the voltage applied to the transducer. In the latter case, an additional effect appears: the polarization of the light in all diffraction orders in general becomes elliptical and, at suitable acoustic frequency and amplitude, circular polarization is possible in the zero order of diffraction. Experimental results are included in the case of argon laser light (lambda = 0.488 mu m) being diffracted by a 48.5 MHz sound wave propagating in fused SiO2. Comparison between the theoretically predicted values and the experimental measurements made for the intermediate diffraction regime confirm the theoretical model.