The second postulate of Special Relativity says:
As measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body.
But isn't it also valid for sound waves? I mean, a jet flying above the speed of sound will not generate waves that travel at V+U (where V is the jet's velocity and U the speed of sound), but waves that travel at U. Provided that Maxwell's laws show that light is a wave, what's the point of this postulate?