  As a non-trivial check of the non-supersymmetric gauge/gravity duality, we
use a near-extremal black brane background to compute the retarded Green's
functions of the stress-energy tensor in N=4 super-Yang-Mills (SYM) theory at
finite temperature. For the long-distance, low-frequency modes of the diagonal
components of the stress-energy tensor, hydrodynamics predicts the existence of
a pole in the correlators corresponding to propagation of sound waves in the
N=4 SYM plasma. The retarded Green's functions obtained from gravity do indeed
exhibit this pole, with the correct values for the sound speed and the rate of
attenuation.
