The longitudinal acoustic phonon dispersion of polycrystalline cobalt was determined by inelastic x-ray scattering up to 99 GPa, throughout the entire stability field of the hcp phase. The obtained aggregate com- pressional and shear sound velocities are compared with recent single crystal results, impulsive stimulated light scattering and ambient pressure ultrasonic measurements, as well as first principle calculations. We observe a linear evolution of the sound velocities with density up to 75 GPa. In this pressure range, the aggregate elastic properties of the polycrystalline sample are reproduced within 3% by a Voigt-Reuss-Hill average of the single crystal Ci j . Above 75 GPa both aggregate velocities show a softening. Our comparative analysis of single- crystalline and polycrystalline results points towards a magnetic origin of the anomaly.