Sometime last year, I was listening to a podcast episode of the Thomistic Institute. I don't recall the exact topic of the presentation, but the speaker was developing for his audience the different related meanings of the term "word." He was going through the idea that a word is, first and foremost, an immaterial idea, thought, or concept in the mind. By the action of the voice, the word is expressed into a material/physical reality as a collection of sounds. The collection of sounds is what we normally think of as a "word," and that's where the speaker started off, but he was getting to a deeper point about the sounds being for the purpose of carrying the content of the immaterial/conceptual word to others who, by receiving and internalizing the sounds, then have that same thought or concept in their own minds.
While he was speaking, all at once, things fell into place for me to understand Mary and her role in salvation and relationship with God and with us according to a new title. Before stating the title itself, let me develop the idea behind it a little.
We accept as revealed truth (because John the Evangelist tells us) that the human person Jesus is actually the divine Person Who is the Word of God, and Whom we now identify within the Holy Trinity as the Son. In His divine, eternal nature, the Son is immaterial. He is the Father's Own interior reflection of Himself, "eternally begotten of the Father...true God from true God...consubstantial with the Father." We also read in John that "the Word was made flesh."
Meditating on that phrase, we see a parallel that seems deeper than mere analogy or even metaphor: As the mental words in our minds are made into physical sounds, so the Word of the Father was made into the material expression of flesh. But there's more to it than that.
We know from Luke that it was by the power of the Holy Ghost that Mary conceived Jesus, the Word, and gave Him his fleshly existence. She was the instrument of the material expression of the Word, but this was through the action of the Holy Spirit that this was achieved. Consider, thought that, the term "spirit" literally means "breath." When theologians think of the eternal act of the Holy Spirit's procession, they often refer to the process as "spiration," as though God the Father is breathing out the Holy Spirit.
Returning, then to the consideration of words, when we speak a word we use the instrument of our voice under the power of our breath to give material expression to an immaterial concept in our mind. This is exactly what the Father did with Mary: He used Mary instrumentally, by the power of the Holy Spirit, to give material expression to His immaterial Word, the Son.
In this way, we can say that Mary is the "Voice of God."
But I believe this is not just true analogically or metaphorically. I believe this is a very real aspect of mariology that has yet to be developed. Consider the Holy Scripture: There is an identity between Jesus and Holy Scripture. Both are the Word of God, given expression into our world. In Holy Scripture, the Word is expressed as writings by the inspired (there's that word "spirit" again) authors who were acting under the power of the Holy Spirit. Then, in the fulfillment of God's complete plan, in a singular act of "inspiration" by the Holy Spirit, Mary expressed the Word as flesh—not actively, but passively as an instrument. Consider, further, the idea of the material spoken word as being for the purpose of being received by others for the purpose of planting in them the immaterial concept. This is exactly what Jesus does for us in the Holy Eucharist: He makes it so that we can receive His material expression in the flesh so that He can dwell with us in a spiritual, immaterial way.
But the English "Voice of God" doesn't quite do it. There's a nuance that needs to be expressed, that might be better brought over in Latin. When we use the phrase "voice of..." in English, we generally are referring to an active exercise of authority. For example, an ambassador, when visiting a foreign head of state, speaks with the voice of his own head of state—he exercises the authority to negotiate, make demands, etc. Mary's place as the "Voice of God" doesn't carry that same sense. Jesus gave authority to the Apostles, and in a special way to Peter. Mary's role, while more complete, is more passive.
I believe this nuance can be well-expressed by extending Latin a bit. The word for king is "rex" (nominative) and the genetive is "regis." We transform this into the feminine analog, regina, which means "queen." Following the same formula, voice is "vox," and its genetive is "vocis." Therefore, I propose as a true title for Mary: "Vocina Dei."