We have determined the surface acoustic wave velocity in InxGa1-xN layers for 0.34 < x < 0.75 by means of high resolution Brillouin spectroscopy. The sagittal dependence of the surface acoustic velocity has been analyzed by comparing the experimental results with theoretical simulations based on the Green's function formalism. We find the best agreement with our data when the bowing parameters for the elastic constants recently reported from density functional theory calculations are taken into account. The dependence of the surface acoustic wave velocity on alloy composition is given. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4744961]