The density dependence of the velocity of sound in liquid mercury along the liquid-vapour coexistence curve is investigated by means of a large-scale molecular-dynamics simulation using the effective pair potential derived from the experimental structure factor by the inverse method. The resulting velocity of sound is in very good agreement with experiment and its density dependence changes at , at which point the metal-nonmetal transition occurs. It is shown that the repulsive part of the effective pair potential plays a crucially important role in the density dependence of the velocity of sound in liquid mercury.