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Strong compositional-dependent elastic properties have been observed theoretically and experimentally in Ti$_{1-x}$Al$_x$ N alloys. The elastic constant, C$_{11}$, changes by more than 50% depending on the Al-content. Increasing the Al-content weakens the average bond strength in the local octahedral arrangements resulting in a more compliant material. On the other hand, it enhances the directional (covalent) nature of the nearest neighbor bonds that results in greater elastic anisotropy and higher sound velocities. The strong dependence of the elastic properties on the Al-content offers new insight into the detailed understanding of the spinodal decomposition and age hardening in Ti$_{1-x}$Al$_x$N alloys.