The voice is a confused sound, discovering no secret of the heart, only signifying that he who utters it desires to say somewhat; it is the word that is the speech that openeth the mystery of the heart. Voice is common to men and other animals, word peculiar to man. John then is called the voice and not the word, because God did not discover His counsels through him, but only signified that He was about to do something among men; but afterwards by His Son he fully opened the mystery of his will.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).