  We provide a unified investigation of normal modes and sound propagation at
zero temperature in Bose condensed gases confined in highly asymmetric harmonic
traps and interacting with repulsive forces. By using hydrodynamic theory for
superfluids we obtain explicit analytic results for the dispersion law of the
low energy discretized modes for both cigar and disk shaped geometries,
including the regime of large quantum numbers where discrete modes can be
identified with phonons. The correspondence with sound propagation in
cylindrical traps and the one-dimensional nature of cigar type configurations
are explicitly discussed.
