I have both of these books, and have decided to review each story in October.
"Dark Music" is a story told from the perspective of a man who has bought a parcel of land in the country to vacation in. After spending some time there, he comes across an old hermit who is living on his land. The hermit tells him to leave, but our narrator is indignant, it is legally his land.
After a few more run-ins, our narrator and the hermit decide to peacefully co-exist, as they are not really interfering with each other. The hermit, named Aaron, becomes fond of our narrator and decides to show him something he's been working on. At night, Aaron brings out a cage of bats, and to our narrator's surprise, they make music, Aaron explaining that he has attached tiny pipes to their wings, making notes when they flap their wings. Even more, he has these bats trained to make music. Our narrator is amazed at this, but Aaron assures him the real surprise is still in store.
One night, Aaron shows our narrator the full scope of what he's been doing. He has a whole swarm of bats with these attachments, trained. Aaron plays a chilling symphony for our narrator, an experience which leaves him unable to enjoy music again, as the sounds are so gruesome he can no longer bear it.
The next morning, our narrator decides to go home, but when he goes to say good-bye to Aaron, he discovers the bats turned on him and ate him alive. Disgusted at this, he sets Aaron's cave on fire, also, presumably, killing all the bats. One bat falls out, and he stamps its head with his foot. As he examines it, he discovers there are no pipes on the wings.
I was standing with the the bat in my hand, so that the golden-green luminance of the glen shone full upon it. I looked more closely at the creature's face. Clots of half-dried blood were darkly smeared about its mouth and nostrils—human blood—Old Aaron's blood. I felt faintly ill. Suddenly, the bat's mouth gaped open. It was not dead, as I had supposed.
And then occurred the final horror, the ultimate loathsomeness. As the bat's mouth gaped wide, I couldn't escape seeing that deep in its mouth, protruding from its throat—inserted there with what must have been astounding skill so that it formed an integral part of the bat's breathing apparatus—was one of Old Aaron's tiny pipes.
While my skin crawled, the bat breathed its death gasp. From the little pipe there burst an obscene bleat of sound that was an echo of the demoniac symphony that had shrieked through the glen the night before.