The P-wave modulus is an elastic modulus, not often used in rock physics interpretation. It is usually denoted M.
M is analogous to the shear modulus, μ and is most easily understood in acoustic terms:
Therefore Mρ is equal to IP2, so one could invert for it relatively easily, as one does for λρ. One could consider M as a cousin to bulk modulus, K, in the same way that the 1st Lamé parameter is a cousin to shear modulus, μ.