Because what you get there is only a copyright in the sound recording, not the underlying musical work. So you can only control uses of your audio recording, yo

Because what you get there is only a copyright in the sound recording, not the underlying musical work. So you can only control uses of your audio recording, you can't stop people from using the same work which is as you noted, in the public domain. By your own logic, the only copyright the edits would entitle one to would be what the edits consisted of and none of the underlying work. This is obviously meaningless in the context of images.