A theoretical study of the vibrational density of states of amorphous alloys was developed by applying a simple effective-medium theory with a Born Hamiltonian. This theory is equivalent to the coherent-potential approximation, with the advantage that it is set in real space in a very simple way. With this theory, one can observe the differences in the spectra of amorphous Si and Ge when several concentrations of N atoms are added. The predictions of the theory are compared with data from Raman scattering experiments.