The global carbon cycle may involve iron carbide as a component of the Earth's inner core. In this study, we report the first experimental measurements of phonon density of states, sound velocities, and Debye temperatures of Fe3C up to 47 GPa and 1450 K using the nuclear resonant inelastic X-ray scattering (NRIXS) method in a laser-heated diamond anvil cell. Within the investigated pressure and temperature range, the effect of temperature on compressional velocity vp falls within the measurement uncertainties. The shear velocity vs decreases with increasing temperature in a non-linear fashion: at ~15 GPa, vs decreases by ~12% between 300 K and 740 K; At ~46 GPa, temperature produces a ~5% reduction between 300 and 960 K and a ~6% further reduction between 960 K and 1370–1450 K. The high-temperature vs of Fe3C deviates from Birch's law behavior towards smaller values, potentially matching the anomalously low vs in the inner core under the relevant pressure and temperature conditions. Our results support Fe3C as a major component of the inner core.