Both are found in What Luther Says, p. 983:
The Devil Detests Melodious Music
We know that music is hateful and intolerable to devils. I firmly believe, nor am I ashamed to assert, that next to theology no art is equal to music; for it is the only one, except theology, which is able to give a quiet and happy mind. This is manifestly proved by the fact that the devil, the author of depressing care and distressing disturbances, almost flees from the sound of music as he does from the word of theology.
This is the reason why the prophets practiced music more than any art and did not put their theology into geometry, into arithmetic, or into astronomy, but into music, intimately uniting theology and music, telling the truth in psalms and songs.
But why do I praise music now, trying to depict--or rather to disfigure--so great a subject on so small a slip of paper? But my love for music, which has often refreshed me and set me free from great worries, abounds and bubbles over.
Oppose Satan and Sadness with Music
When sadness comes to you and therefore threatens to gain the upper hand, then say: Come, I must play our Lord Christ a song on the organ (be it the Te Deum laudaumus or the Benedictus); for Scripture teaches me that He loves to hear joyful song and stringed instruments. And strike the keys with a will, and sing out until the thoughts disappear, as David and Elisha (1 Sam. 16:23, 2 Kings 3:15) did.
If the devil returns and suggests cares or sad thoughts, then defend yourself with a will and say: Get out, devil, I must now sing and play to my Lord Christ.