They are addressing decomp. Since they are "dead" there will come a point and time to where the muscles do not work because of the putrefaction factor. So the fact they are all still able to walk and move would suggest that the virus/bacteria or what ever it is is mutating(evolving)...the voice heard was just too eerily close to how the walkers en masse groan. So if the Whisperers are voice/sound impersonator masters as well then so be it. If I am over thinking it...typical.