I suspect that this was intended to be an amusing question and little more, but I would imagine that a failed Move Silently roll on the part of the living spell would represent not so much the creature itself causing a noise but rather the creature blocking a previously-noisy area/object. This would cause the observer to suddenly cease hearing the noise, and be able to ascertain the general location of the beast. So, to answer your question, I suspect a living silence suddenly ends some of the local ambient noise when it fails such a check.