  An interesting probe of the nature of dark energy is the measure of its sound
speed, $c_s$. We review the significance for constraining sound speed models of
dark energy using large neutral hydrogen (HI) surveys with the Square Kilometre
Array (SKA). Our analysis considers the effect on the sound speed measurement
that arises from the covariance of $c_s$ with the dark energy density,
$\Omega_\LLambda$, and a time-varying equation of state, $w(a)=w_0+(1-a)w_a$.
We find that the approximate degeneracy between dark energy parameters that
arises in power spectrum observations is lifted through redshift tomography of
the HI-galaxy angular power spectrum, resulting in sound speed constraints that
are not severely degraded. The cross-correlation of the galaxy and the
integrated Sachs-Wolfe (ISW) effect spectra contributes approximately 10
percent of the information that is needed to distinguish variations in the dark
energy parameters, and most of the discriminating signal comes from the galaxy
auto-correlation spectrum. We also find that the sound speed constraints are
weakly sensitive to the HI bias model. These constraints do not improve
substantially for a significantly deeper HI survey since most of the clustering
sensitivity to sound speed variations arises from $z \lsim 1.5$. A detection of
models with sound speeds close to zero, $c_s \lsim 0.01,$ is possible for dark
energy models with $w\gsim -0.9$.
