The "ear" of God are the Laws, the Commandments of God, which "hear" man, i.e. always work: they protect against the commission of sin, protect against the error of the one who does them, and punish the consequences of the sins of the one who violates them.

The "ear" of God is God's "perception," God's measuring by the action of His Laws, the Commandments, of the conformity of man's way of life to those Laws.
It is His omnipresent Spirit of the Wisdom of the Will of God, manifested in all the Laws of Being, and thus open, accessible, able to "hear", i.e. to perceive the prayer appeal of man.
The "ear" of God also symbolizes the intellect and consciousness, the knowledge of man as a part of the original and permanent presence of God in man. 
The "ear" of man is a symbol of man's ability through his mind and conscience to "hear", i.e. to perceive, realize the Word of God, His Will, His Commandments.