  We consider a system of one-dimensional fermions moving in one direction,
such as electrons at the edge of a quantum Hall system. At sufficiently long
time scales the system is brought to equilibrium by weak interactions between
the particles, which conserve their total number, energy, and momentum. Time
evolution of the system near equilibrium is described by hydrodynamics based on
the three conservation laws. We find that the system supports three sound
modes. In the low temperature limit one mode is a pure oscillation of particle
density, analogous to the ordinary sound. The other two modes involve
oscillations of both particle and entropy densities. In the presence of
disorder, the first sound mode is strongly damped at frequencies below the
momentum relaxation rate, whereas the other two modes remain weakly damped.
