Understanding the alloying effects of nickel and light element(s) on the physical properties of iron under core conditions is crucial for interpreting and constraining geophysical and geochemical models. We have studied two alloys, Fe0.92Ni0.08 and Fe0.85Si0.15, with nuclear resonant inelastic x-ray scattering up to 106 GPa and 70 GPa, respectively. The sound velocities of the alloys are obtained from the measured partial phonon density of states for 57Fe incorporated in the alloys. Addition of Ni slightly decreases the compression wave velocity and shear wave velocity of Fe under high pressures. Silicon alloyed with Fe increases the compressional wave velocity and shear wave velocity under high pressures, which provides a better match to seismological data of the Earth's core.