To a great extent, the voices that Annie hears represent her own sense of fear, insecurity, and doubt.  The voices operate to galvanize her into action, providing her the internal motivation to face the task of teaching Helen in its most direct manner.  These voices, especially the voices of her younger brother, help to give insight into her motivation.   When they are silenced by the end of the play, a couple of elements are revealed.  The first is that Annie has gone very far in replacing these voices with a sense of accomplishment and a sense of totality.  As Annie teaches Helen, these voices operate as representation of the incompleteness in Annie's being.  As she has accomplished the fundamental goal of teaching Helen, becoming "the miracle worker," the absence of these voices signifies a sense of coherency and unity is evident.  Another significant element that emerges when these voices disappear is the idea that Annie's guilt over her role in her brother's death has also dissipated.  Annie clutches Helen as she did her brother, and the disappearance of these voices helps to bring out that Annie has both found her calling and established a sense of peace and tranquility that emerges with what has taken place.  To this extent, the significance of the lack of voices represents accomplishment in this domain.