We calculate the optical loss of composite materials, formed from statistically distributed metal nanoclusters embedded in an optically absorbing matrix material. The method includes the generation of cluster assemblies using a Monte-Carlo approach to account for the statistical distribution of the clusters with respect to size, shape and arrangement, which is specific to the sample production procedure. Subsequently, the optical loss is calculated within the framework of the classical electromagnetic theory in terms of the optical response of spherical clusters. The effects of the electromagnetic interaction between the clusters in the assembly are described based on the solution of the multiple scattering problem in non-absorbing host media (generalized Mie theory) extended to take into account absorption in the matrix In contrast to conventional optical mixing models our treatment considers both absorption and scattering losses. We demonstrate the qualitative agreement of the calculated loss coefficient with experimental data.