Invariance of the acoustic wave equation under transformed Galilean transformation

The principle of relativity requires that laws be invariant in all inertial reference frames. The laws of mechanics are invariant to Galilean relativity. The acoustic wave equation is a mechanical law. So why does the acoustic wave equation turn out to be noninvariant to Galilean transformation? What does this mean? Why do the principle of relativity, the wave equation, and the current Galilean transformation not agree between them? Indeed, to provide the invariance of the wave equation, the Galilean transformation must be transformed. The transformed Galilean transformation has a wide base of arguments.