We consider coupled systems of semi-linear wave equations with different sound speeds on a finite time interval [0, T ] and a bounded Lipschitz domain Ω in R 3 with boundary ∂Ω. We show the coupled systems are well posed for variable coefficient sounds speeds and short times. Under the assumption of small initial data, we prove the source to solution map associated with the nonlinear problem is sufficient to determine the source to solution map for the linear problem. We can then reconstruct the sound speeds in Ω for the coupled nonlinear wave equations under certain geometric assumptions. In the case of the full source to solution map in Ω × [0, T ] this reconstruction could also be accomplished under fewer geometric assumptions.