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A model is presented which for the first time provides a description of the long-wavelength phonons in mixed crystals of the type A1B (1-x) C (x) as a system of two coupled models: a lattice mode in a virtual crystal approximation (VCA) and a localized mode of the disordered sub-lattice. These two modes interact by effects beyond the VCA, namely by the difference of the vibrational frequencies of the atoms B and C perceived by the atom A. Two cases will occur: (i) The two modes interact only weakly over the entire composition range. Then a lattice-like mode and in addition a mode in which the disordered sub-lattice is nearly involved alone are present. (ii) The two modes become resonant at a certain composition and interact strongly. This leads to the two-mode behaviour. The dielectric function assumes a form typical for the presence of two oscillators for both types of mode behaviour.