“With my voice have I cried unto the Lord” (Psalm 3:4); that is, not with the voice of the body, which is drawn out with the sound of the reverberation of the air; but with the voice of the heart, which to men speaks not, but with God sounds as a cry. By this voice Susanna was heard (Daniel 13:44); and with this voice the Lord Himself commanded that prayer should be made in closets,(Matthew 6:6) that is, in the recesses of the heart noiselessly.

(St. Augustine: Expositions on the Book of Psalms, Psalm 3)

In his commentary on Psalm 3, St. Augustine describes this prayer as “the voice of the heart”. He supports his position with quotes from 6 passages of Sacred Scripture (I will mention 2 here): First in the Book of Daniel, where Susanna, who is falsely accused of adultery, “Then Susanna cried out with a loud voice” (Daniel 13:42).  Also, St. Augustine ties this voice of the heart to the Sermon on the Mount, where Jesus teaches “But when you pray, go into your room and shut the door…”.  Here is how St. Augustine describes this type of prayer: “this is called a cry by reason of the strength of its intention.”