Sound velocity measurements on candidate mantle minerals at relevant mantle conditions are needed to interpret Earth's seismic structure in terms of model abundances, variable composition, and other potentially influential parameters such as electronic spin-pairing transitions. Here the sound velocities of the lower-mantle ferropericlase have been measured by nuclear resonant inelastic X-ray scattering to 110 GPa. Compressional and shear wave velocities and their pressure derivatives rise dramatically across the spin-pairing transition of iron in (Mg0.75Fe0.25)O above 50 GPa. Effects of the transition on the sound velocities of (Mg, Fe)O at lower-mantle pressures yield values that are much greater than what is predicted by studying pure MgO and high-spin ferropericlase. Our results indicate that sound velocities of the low-spin ferropericlase need to be considered in future geophysical and geochemical models, which could offset the effect of the addition of iron in the lower-mantle minerals and affect the evaluation of the lower-mantle heterogeneities.