A systematic method of approximation is studied in this series of work in order to find the ensemble-averaged Green function ≪G> which describes one-electron properties of a substitutionally disordered lattice. In a previous article, an expression for the self-energy has been derived within a single-site approximation on rigorously including the “exclusion effect”, or the “kinematic correction”. The result turned out to be equivalent to the so-called coherent potential approximation (CPA). In the present article, a double-site approximation or an extension of the CPA to the case with pair effects is made and applied to the investigation of the phonon Green function in an isotopically disordered system for the purpose of the numerical calculation of the frequency spectrum. When the calculation is performed on the basis of the density of states solved exactly for a perfect lattice, the first-order approximation (the CPA) does not yield a spiky structure. On the other hand, when the double-site correction is taken into account, the calculated phonon spectrum is improved and the fine structure and a tail appear in the spectrum.