Answer: Waves other than Electromagnetic Waves require a medium and hence for each one of them there exists a special frame namely the one where the medium at infinity is at rest. Thus the velocity of such waves is not the same in each inertial frame and that presents no violation of the Principle of Relativity. The EM waves require no medium, the velocity of propagation is the same in all inertial frames and the laws of transformations contained in the Special Theory of Relativity respect this unique wave.
