  It has been conjectured that the velocity of sound in any medium is smaller
than the velocity of light in vacuum divided by $\sqrt{3}$. Simple arguments
support this bound in non-relativistic and/or weakly coupled theories. The
bound has been demonstrated in several classes of strongly coupled theories
with gravity duals and is saturated only in conformal theories. We point out
that the existence of neutron stars with masses around two solar masses
combined with the knowledge of the equation of state of hadronic matter at
"low" densities is in strong tension with this bound.
