Why were the pirates afraid when they heard singing ?

The pirates thought it was a ghost. Finally, the buccaneers reach the top of the steep plateau, the men sit down and look at the view. As the men whisper about Captain Flint's exploits, they suddenly hear a voice singing his favorite song:

Fifteen men on the dead man's chest

Yo-ho-ho, and a bottle of rum!

Terrified, the pirates turn pale. But as suddenly as the voice began, it brakes off. Only slowly, Long John Silver tells the rest of the group that the voice is not a ghost and that his voice seems familiar.