To constrain the interior structure of terrestrial planets and in particular the composition their cores, elastic properties (density and bulk modulus) of liquid iron alloys at high pressure are essential. In this study, we measured sound velocity and density of liquid Fe-Ni-S and Fe-Ni-Si at high pressures using large volume press with ultrasonic and X-ray absorption methods and modeled the core compositions of Mercury and Mars. Our results show that estimated amounts of S or Si in the Mercury core are comparable with the previous estimates whereas for Mars, the predicted amount of S is larger.