13 “The Lord has heard the longing of the poor”: that longing wherewith they were burning, when in the straits and tribulations of this world they desired the day of the Lord. “Your ear has heard the preparation of their heart.” This is the preparation of the heart, of which it is sung in another Psalm, “My heart is prepared, O God, my heart is prepared:” of which the Apostle says, “But if we hope for what we see not, we do with patience wait for it.” Now, by the ear of God, we ought, according to a general rule of interpretation, to understand not a bodily member, but the power whereby He hears; and so (not to repeat this often) by whatever members of His are mentioned, which in us are visible and bodily, must be understood powers of operation. For we must not suppose it anything bodily, in that the Lord God hears not the sound of the voice, but the preparation of the heart.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)