We have carried out nuclear resonant scattering measurements on 57Fe-enriched Fe3C between 1 bar and 50 GPa at 300 K. Synchrotron Mo¨ssbauer spectra reveal a pressure-induced magnetic transition in Fe3C between 4.3 and 6.5 GPa. On the basis of our nuclear resonant inelastic X-ray scattering spectra and existing equation-of-state data, we have derived the compressional wave velocity VP and shear wave velocity VS for the high-pressure nonmagnetic phase, which can be expressed as functions of density (r): VP(km/s) = -3.99 + 1.29r(g/cm3) and VS(km/s) = 1.45 + 0.24r(g/cm3). The addition of carbon to iron-nickel alloy brings density, VP and VS closer to seismic observations, supporting carbon as a principal light element in the Earth’s inner core.