Bogoliubov speed of sound for a dilute Bose-Einstein condensate in a 3d   optical lattice
  We point out that the velocity of propagation of sound wavepackets in a Bose-Einstein condensate filling a three-dimensional cubic optical lattice undergoes a maximum with increasing lattice depth. For a realistic choice of parameters, the maximum sound velocity in a lattice condensate can exceed the sound velocity in a homogeneous condensate with the same average density by 30%. The maximum falls into the superfluid regime, and should be observable under currently achievable laboratory conditions. 
Dave Boers, Christoph Weiss, Martin Holthaus