  Disorder modifies the sound-wave excitation spectrum of Bose-Einstein
condensates. We consider the classical hydrodynamic limit, where the disorder
correlation length is much longer than the condensate healing length. By
perturbation theory, we compute the phonon lifetime and correction to the speed
of sound. This correction is found to be negative in all dimensions, with
universal asymptotics for smooth correlations. Considering in detail optical
speckle potentials, we find a quite rich intermediate structure. This has
consequences for the average density of states, particularly in one dimension,
where we find a "boson dip" next to a sharp "boson peak" as function of
frequency. In one dimension, our prediction is verified in detail by a
numerical integration of the Gross-Pitaevskii equation.
