Those of us readers who are not madmen must realize that it could not be the old man's heart the narrator is hearing. This is why:
First of all I dismembered the corpse. I cut off the head and the arms and the legs.
Poe has his narrator include these words in his description of how he concealed the body because Poe wants the reader to be quite sure the old man is dead. After all, the narrator has cut off the head. He also refers to the victim as a "corpse." If we take it for granted that the old man is totally dead, then it could not be his heart that the narrator imagines he is hearing.
However, since the murderer is mad, it is not impossible that he should imagine it is the old man's heart he keeps hearing. The answer seems to be that he thinks he is hearing the victim's heart and we know he is hearing his own heart. His agitated behavior towards the end of the story strongly suggests that his heart must be pulsating abnormally.
I arose and argued about trifles, in a high key and with violent gesticulations; but the noise steadily increased. Why would they not be gone? I paced the floor to and fro with heavy strides, as if excited to fury by the observations of the men—but the noise steadily increased. Oh God! what could I do? I foamed—I raved—I swore! I swung the chair upon which I had been sitting, and grated it upon the boards, but the noise arose over all and continually increased.
Obviously he is increasing his own blood pressure, his own heartbeat, his own pulse, and the flow of his own adrenaline with his exertions and his hallucinations. He is giving himself away. We have all experienced such feelings--though not as strongly, and we have not had dismembered bodies buried under our floors. The fact is that other people can't read our feelings as well as we think they can. The three police officers are not "dissembling." They are not "making a mockery" of the narrator. And the narrator is probably not behaving as wildly as he seems to imagine. Just moments earlier he had described the officers as follows:
The officers were satisfied. My manner had convinced them. I was singularly at ease. They sat, and while I answered cheerily, they chatted of familiar things.
So the title of the story, "The Tell-Tale Heart," probably refers both the narrator's heart and the old man's tell-tale heartbeat which he imagines.