Final answer:
The specific sound that frightened Casey and Margaret is not mentioned in the passages provided.
Step-by-step explanation:
Casey and Margaret heard a sound that frightened them, but the specific sound is not mentioned in the passages provided. In the first passage, Mr. and Mrs. Hadley screamed, which made Casey and Margaret realize why the other screams sounded familiar. In the second passage, Venters heard a low, rumbling roar, but Bess did not hear anything.
In the third passage, Bess mentioned a sound called Oldring's knell, which was a gale blowing in the caves. However, it is not explicitly stated that Casey and Margaret heard this sound. In the fourth passage, Jane is terrified by the silence, but there is no mention of a specific sound. In the fifth passage, John shouted that they needed to get out before the chateau was shelled, but this does not directly relate to Casey and Margaret's fear.
In the sixth passage, a low screaming or grating sound is mentioned, but it is not clear if Casey and Margaret heard this sound. In the seventh passage, the sound of shells and gunfire is mentioned, but again, it is not specified if Casey and Margaret heard this sound.
In the eighth passage, the sound of dropping bars inside the barn is mentioned, but it is unsure if this is the sound that frightened Casey and Margaret. In the ninth passage, John heard a faint far-away sound, followed by a closer noise, but it is not clear if this is related to Casey and Margaret's fear.