Sound propagation velocity of surface acoustic waves (SAWs) and bulk acoustic waves (BAWs) has been investigated by means of high resolution Brillouin spectroscopy. The results show a linear dependence of the BAW propagation velocity with the Al concentration. There is no relevant influence of the substrate chosen in the propagation velocity of BAWs in AlxGa1-xN thin films. SAW propagation velocity of epitaxially grown AlN is clearly lower than the observed one in AlN prepared by reactive dc magnetron sputtering. Numerical simulation results of SAW propagation velocity are compared with the experimental results. (C) 2002 American Institute of Physics.