There is no restriction other than $c_s<c$. Relativistic plasmas and fluids explore this regime. A weakly coupled quark gluon plasma has $c_s=c/\sqrt{3}$. Even higher speeds are reached in neutron stars, see Is the speed of sound almost as high as the speed of light in neutron stars? .
The speed of sound is related to the adiabatic compressibility $$ c_s^2 = \left(\frac{\partial P}{\partial \rho}\right)_s \, . $$ This quantity also enters the neutron structure via the TOV equation. It constrains, in particular, the maximum mass and the mass-radius relation. The recent observation of a 2-solar mass neutron star implies that $c_s$ becomes quite large, certainly bigger than $0.5c$.
Adendum: See here for a more quantitative analysis based on the existence of a 2 M(solar) neutron star, and equ.(10) of this paper for a theoretical limit in which we can shows that $c_s\to c$.
