The Amulet of Humanspeak is referenced in the Recipe for Disaster quest. When Awowogei is unable to converse with the other feast attendees, Lumbridge Sage comments that he must have 'lost his amulet of humanspeak'. Also, in The Curse of Zaros, if you talk to the ghost at the wrong location, you will sometimes say "Wooo woooooo woooooooooo wooo," which are the words that a ghost will say if you do not wear a Ghostspeak amulet. When the ghost does not understand you, your character will comment that the ghost isn't wearing an "Amulet of Humanspeak".