9 And what remains? What remedy amid such ills, in such temptations, such dangers? “I said unto the Lord, You are my God”. Loud is the voice of prayer, it excites confidence. Is He not the God of the others? Of whom is not He God, who is the true God? Yet is He specially theirs, who enjoy Him, who serve Him, who willingly submit to Him. For the wicked too, though unwillingly, are subject to Him....“Hear with Your ears the voice of my prayer.” He did not say, “Hear with Your ears my prayer;” but, as though expressing more plainly the affection of his heart, “the voice of my prayer,” the life of my prayer, the soul of my prayer, not that which sounds in my words, but that which gives life to my words.
For all other noises without life may be called sounds, but not words. Words belong to those that have souls, to the living. But how many pray to God, yet have neither perception of God, nor right thoughts concerning God! These may have the sound of prayer, the voice they cannot, for there is no life in them. This was the voice of the prayer of one who was alive, forasmuch as he understood that God was his God, saw by Whom he was freed, perceived from whom he was freed.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)