I'm a physicist and I have absolutely no idea what you mean by an "electromagnetic audio signal". I think what you mean is an EM wave which happens to carry an

I'm a physicist and I have absolutely no idea what you mean by an "electromagnetic audio signal". I think what you mean is an EM wave which happens to carry an amplitude-modulated audio signal , which is not an audio wave. Audio waves are compressional waves formed in a medium, and require a medium for transmission through, such as air, or rock, or water, or even interstellar gas. Electromagnetic waves require no medium, and are brought about by energy transference between electric and magnetic fields in free space.