  The propagation of the isoscalar and isovector sound modes in a hot nuclear
matter is considered. The approach is based on the collisional kinetic theory
and takes into account the temperature and memory effects. It is shown that the
sound velocity and the attenuation coefficient are significantly influenced by
the Fermi surface distortion (FSD). The corresponding influence is much
stronger for the isoscalar mode than for the isovector one. The memory effects
cause a non-monotonous behavior of the attenuation coefficient as a function of
the relaxation time leading to a zero-to-first sound transition with increasing
temperature. The mixing of both the isoscalar and the isovector sound modes in
an asymmetric nuclear matter is evaluated. The condition for the bulk
instability and the instability growth rate in the presence of the memory
effects is studied. It is shown that both the FSD and the relaxation processes
lead to a shift of the maximum of the instability growth rate to the longer
wave length region.
