Ultrasonics has a rich history in measuring state changes of materials.  However, the theory behind related measurements is often qualitative especially when elastic waves have been used to probe phase evolution and precipitation in solids.  While linear coupling of elastic deformations and solute diffusion in solids produces effects such as strain-induced diffusion and solute-induced dilatation, there are significant physical effects that must be described using the nonlinear coupling of solute concentration to elastic stiffness.  We have recently developed nonlinear continuum models that consider the possible effects that might arise in elastic solids containing mobile solute species and have applied these models to understand solute concentration effects on elastic wavespeeds during precipitation processes in aluminum alloys.