I don't need help with a numerical solution here - mostly a concept check. I've been asked to calculate the speed of sound in metallic sodium at T = 0 K using Fermi-Dirac statistics. After doing so, I get a speed of 14.5 meters per second, which is, well, really slow. I would have expected sound to travel much more quickly than in air at that temperature. Is my answer likely to be right? If so, could someone explain why the result is so counter-intuitive?