Sound speed for a BEC in a 2D optical lattice via the strength of the optical lattice. The numerical results are denoted by the solid lines, analytical results for weak potentials by stars, and analytical tight-binding results by solid squares. Sound speed of vs,x is in units of 2kL/m, the nonlinear coefficient c=n0as/kL2 , and the lattice strength of v is in units of 42kL2/m.