The police officers didn't hear the beating heart hidden under the floorboards of the living room. It was an hallucination by the narrator who anticipated getting caught for the murder of the old man. The police officers were calm because they were just on a preliminary investigation for a disappearance and didn't expect a murder had been committed. It was all in the narrator's mind that they suspected him of murder. Throughout the "interrogation" of the narrator, he was getting more and more agitated at the officers' apparent calmness. He thought that they, too, should be able to hear the beating heart. The narrator probably could have gotten off and the dead man's heart never found if he had not panicked and eventually confess. It was his own insanity or guilt that was his undoing. The officers really just stumbled onto the crime scene and didn't expect the frantic behavior of the narrator.