Results of measurements of sound velocity and attenuation in fluid mercury at sub- and supercritical parameters of state (at temperatures up to 2100 K and pressures up to 190 Mpa) are reviewed. Discussion is concentrated on anomalies observed near the liquid-gas critical point. These are: (1) peculiarities in sound velocity and attenuation revealed in the close vicinity of the critical point, which can be attributed to an effect of fluctuation-induced forces in a prewetting film on the cell walls. (2) A maximum in sound attenuation in a region of a metal-dielectric transition, testifying for fluctuations accompanying the opening of a band gap. (3) A pressure dependence of sound velocity in superheated vapor, which changes sign near the critical temperature.