We study tile sound propagation in charge- and spin-density wave syste
ms in the hydrodynamic limit. For the longitudinal sound wave the long
-range Coulomb interaction plays a crucial role in suppressing charge
fluctuations. We find that the longitudinal wave does not couple to th
e phason (i.e. no electromechanical effect) independent of the directi
on of the polarization vector u, while the transverse wave couples to
the phason only if u is parallel to the chain direction. This describe
s at least qualitatively a peculiar asymmetry found recently in sound
propagation in (TaSe4)(2)I.