These audio books are in public domain.
(See recording details behind the links.)
Martin Luther
The Small Catechism
The Large Catechism
Concerning Christian Liberty
The Bondage of The Will (part I)
The Bondage of The Will (part II)
The Smalcald Articles
On Babylonian Captivity of The Church
Philipp Melanchthon
The Augsburg Confession
The Defense of The Augsburg Confession (part I)
The Defense of The Augsburg Confession (part II)
A Treatise on the Power and Primacy of the Pope
Jacob Andreae and Martin Chemnitz
The Epitome of The Formula of Concord
Solid Declaration of the Formula of Concord
Catalog of Testimonies
Johann Gerhard
Sacred Meditations
C. F. W. Walther
The Proper Distinction Between the Law And the Gospel (part I)
The Proper Distinction Between the Law And the Gospel (part II)
The Proper Distinction Between the Law And the Gospel (part III)