  It has been suggested in the literature that the sound velocity of the
nuclear matter $v_s$ violates the so-called sound velocity bound $v_s \le
c/\sqrt{3}$ at high density, where $c$ is the speed of light. In this paper, we
revisit this issue and confront the current measurements of mass, radius, and
tidal deformability of neutron stars with $10^5$ different equations of state
which are parametrized at low density and saturates the sound velocity bound
beyond twice the saturation density where the equation of state has not been
constrained yet, by which we can conservatively obtain the maximum mass of the
neutron stars compatible both with the observed properties of neutron stars and
the sound velocity bound. We find that majority of the models are eliminated by
the incompatibility with the observations and, especially, the recently
detected massive pulsar ($2.35\pm 0.17 M_\odot$) is hardly realized by our
simulations. Our study strongly supports the violation of the sound velocity
bound.
