  Sound propagation in superfluid $^3$He in aerogel is studied on the basis of
a two-fluid model taking into account the effect by the drag force due to
collisions between $^3$He-quasiparticles and aerogel molecules. The drag force
plays a role of frictional force between the aerogel and the normal-fluid
component. In local equilibrium, they move together in accordance with McKenna
{\it et al.}'s model. The deviation from the local equilibrium leads to the
damping of sound. We give explicit expressions for the attenuation of
longitudinal sounds in this system. We also discuss the sound propagation in a
superfluid ${}^3$He-aerogel system embedded in a narrow pore. It is shown that
the forth sound propagates in such a system because of the clamping of the
normal fluid by the aerogel.
