  In this paper we study the sound speed - $c_s^2$, which is directly related
to the classical perturbations - of the dynamical dark energy (DE), especially
with an equation of state crossing the cosmological constant boundary in
details and show its implications on Cosmic Microwave Background (CMB)
Anisotropy. With the present observational data of CMB, Type Ia Supernova
(SNIa) and galaxy clustering, we perform a global analysis to constrain the
sound speed of dark energy, using the Markov Chain Monte Carlo method. We find
that the sound speed of dark energy is weakly constrained by current
observations thus the futuristic precision measurements of CMB on a very large
angular scale (low multipoles) are necessary.
