  We study a model of flocking for a very large system (N=320,000) numerically.
We find that in the long wavelength, long time limit, the fluctuations of the
velocity and density fields are carried by propagating sound modes, whose
dispersion and damping agree quantitatively with the predictions of our
previous work using a continuum equation. We find that the sound velocity is
anisotropic and characterized by its speed $c$ for propagation perpendicular to
the mean velocity $<\vec{v}>$, $<\vec{v}>$ itself, and a third velocity
$\lambda <\vec{v}>$, arising explicitly from the lack of Galilean invariance in
flocks.
