The OnPerception event is fired when one creature perceives (or ceases to perceive) another. This single type of event includes both vision (creatures appearing and disappearing) and hearing (creatures becoming audible and inaudible), so it tends to fire twice whenever one creature comes within perception range of another. Being "audible" refers specifically to the listen versus move silently dynamic; it does not include hearing another creature speak. (For detecting speech, use the OnConversation event.)