Today's Gospel is the story of Mary's visit to Elizabeth, and the encounter of the unborn Jesus and his precursor. It's an amazing story; even more amazing when you pair it with other Scripture texts. The liturgy pairs the Gospel with a lovely passage from the Song of Songs. That reading ends with the words "Let me see you, let me hear your voice, for your voice is sweet and you are lovely."
It is the sound of that sweet voice that triggers all the excitement in the Gospel. "When Elizabeth heard Mary's greeting, the infant leaped in her womb and Elizabeth [was] filled with the Holy Spirit..." As holy as the Virgin Mary is, no human voice alone has the power to fill another person with the Holy Spirit. We might go so far as to say that the unborn Savior was speaking through his mother's voice. The grown-up John seems to say, in fact, that what he heard and responded to that day was not so much Mary's voice as the "voice of the Bridegroom" she carried: "The friend of the Bridegroom stands and listens for him, and rejoices greatly at the Bridegroom's voice. That is my joy, and it is full" (Jn. 3).
In turn, the unborn prophet seemed to be speaking through his mother's voice when she said, "Blessed are you who believed that what was spoken to you by the Lord would be fulfilled."
Ultimately, today's Advent liturgy is about being able to listen. Because only if we listen, will we be able to respond, in the words of the Responsorial Psalm, "Our soul waits for the Lord...for in him our hearts rejoice."