Thermoelectric (TE) materials serve for devices converting waste heat into electrical energy as well as for solid-state refrigeration and are, therefore, of utmost technological importance. The energy convergence efficiency of a TE device is determined by the dimensionless figure of merit, ZT. A good TE material having high ZT-values should possess high electrical conductivity and low thermal conductivity; such combination is very challenging from a materials science point of view. We perform lattice dynamics first-principles calculations for the promising AgSbTe₂ TE compound. Based on its calculated vibrational density-of-states, we hypothesize that formation of substitution defects at the Ag-sublattice sites will impede lattice vibrations, thereby reducing the lattice thermal conductivity. Further calculations performed for a La_0.125Ag_0.875SbTe₂-lattice indicate significant reduction of the average sound velocity from 1727 to 1352 m/s due to La-doping. This corresponds to an estimated decrease of lattice thermal conductivity by a factor of 1.63, which is expected to yield a significant improvement of the TE figure of merit for La_xAg_1-xSbTe₂-based materials up to 2.7.