Alloying generally reduces lattice thermal conductivity (k L ) due to increased phonon scattering. Further reduction in k L may result from lattice softening, as observed in SnTe-AgSbTe 2 alloys 35 and Na-doped PbTe. k L can be directly determined from first principles by calculating the second and third-order interatomic force (IFCs) constants . In these calculations, the effects of both phonon scattering and lattice softening are captured. However, in the finite-displacement method, the evaluation of the third-order IFCs requires pairwise atomic displacements to be performed in a chosen supercell, which rapidly scales with the supercell size. For alloys, where the "unit cell" is already a supercell, such calculations are prohibitively expensive. In contrast, the effect of alloying on lattice softening can be assessed from first principles by computing the speed of sound. In this work, we focus on lattice softening. Alloy scattering of phonons will only further reduce k L .