Final answer:
In 'The Hound of the Baskervilles,' Watson hears a mysterious howl on the moor, which Stapleton dismisses as natural sounds, though it is linked to a spectral hound legend.
Step-by-step explanation:
The strange sound that Watson heard in the moor, as described in The Hound of the Baskervilles by Arthur Conan Doyle, was a mysterious, chilling howl that added to the eerie atmosphere of the setting.
Stapleton explains the unsettling sound on the moor as the cries of the moorland birds or the baying of distant dogs, although the true cause is more sinister - connected to the legend of the hound haunting the Baskerville family. The sound plays a crucial role in building the suspense and mystery of the narrative as it progresses.