  We investigate collective excitations coupled with baryon density in a system
of massless three-flavor quarks in the collisionless regime. By using the
Nambu-Jona-Lasinio (NJL) model in the mean-field approximation, we derive the
spectra both for the normal and color-flavor locked (CFL) superfluid phases at
zero temperature. In the normal phase, we obtain zero sound as a low-lying
collective mode in the particle-hole (vector) channel. In the CFL phase, when
the excitation energy, $\omega$, is smaller than the threshold given by twice
the pairing gap $\Delta$ ($\omega<2\Delta$), a phonon corresponding to
fluctuations in the U(1) phase of $\Delta$ appears as a sharp peak in the
particle-particle ($H$) channel. We reproduce the property known from low
energy effective theories that this mode propagates at a velocity of
$v_H=1/\sqrt{3}$ in the low momentum regime; the decay constant $f_H$ in the
NJL model is identical with the QCD result in the mean-field approximation. We
also find that as the momentum of the phonon increases, the excitation energy
goes up and asymptotically approaches $\omega=2\Delta$. Above the threshold for
pair excitations ($\omega>2\Delta$), zero sound manifests itself in the vector
channel. By locating the zero sound pole of the vector propagator in the
complex energy plane we investigate the attenuation and energy dispersion
relation of zero sound. In the long wavelength limit, the phonon mode has its
spectral weight in the $H$ channel alone, while the spectral function vanishes
in the vector channel. We finally extend our study to the case of nonzero
temperature.
