  On the basis of a moment method, general solutions of a linearized Boltzmann
equation for a normal Fermi system are investigated. In particular, we study
the sound velocities and damping rates as functions of the temperature and the
coupling constant. In the extreme limits of collisionless and hydrodynamic
regimes, eigenfrequency of sound mode obtained from the moment equations
reproduces the well-known results of zero sound and first sound. In addition,
the moment method can describe crossover between those extreme limits at finite
temperatures. Solutions of the moment equations also involve a thermal
diffusion mode. From solutions of these equations, we discuss excitation
spectra corresponding to the particle-hole continuum as well as collective
excitations. We also discuss a collective mode in a weak coupling case.
