  The theory of the ultrasound propagation in the mixed state of type-II
superconductors is suggested which takes into account the Magnus force on
vortices, the anti-Magnus force on ions, and diamagnetism of the mixed state.
The acoustic Faraday effect (rotation of polarization of the transverse
ultrasonic wave propagating along vortices) is shown to be linear in the Magnus
force in any regime of the flux flow for wavelengths used in the ultrasound
experiments now. Therefore, in contrast to previous predictions, the Faraday
effect should be looked for only in clean superconductors with a large
amplitude of the Magnus force.
