C.F.W. Walther gives us some insight into why not every sermon (or song, for that matter) that is built from the Scriptures is a true or orthodox sermon.
That is the litmus test of a proper sermon. The value of a sermon depends not only on whether every statement in it is taken from the Word of God and on whether it is in agreement with the same but also on whether Law and Gospel have been rightly distinguished. If the same building materials are provided to two different architects, sometimes one will construct a magnificent building, while the other, using the same materials, will make a mess of it. Because he is dim-witted, the latter may want to begin with the roof, or place all the windows in one room, or stack layers of stone or brick in such a way that the wall will be crooked. One house will be out of plumb and such a bungled piece of work that it will collapse, while the other will stand firm and be a habitable and pleasant place to live. In like manner, two different sermons might contain all the various doctrines–and while the one sermon may be a glorious and precious piece of work, the other may be wrong throughout. …
This frequently happens when students give sermons. [Walther is giving lectures to seminary students.] You will hear comforting remarks such as “It is all by grace,” only to be followed by “We must do good works,” which are then followed by statements such as “With our works we cannot gain salvation.” There is no order to such sermons. Nobody understands them–least of all the person who needs one of these doctrines most.
C.F.W. Walther, Law and Gospel, 37-38
Balaam's Ass 