The correct statement regarding heart sounds is option 3: "Heart sounds are caused by the opening and closing of heart valves."

Option 1 states that heart sounds are caused by the internal blood flow inside the heart. This is incorrect because heart sounds are not caused by the flow of blood, but rather by the movement of the heart valves.

Option 2 states that heart sounds are caused by the external blood flow outside the heart. This is also incorrect because heart sounds are primarily generated within the heart itself, not from external factors like blood flow.

Option 4 states that normal sounds are called `murmurs`. This is incorrect because murmurs refer to abnormal or extra heart sounds, not the normal heart sounds produced by the opening and closing of valves.

Therefore, the correct answer is option 3, as it accurately describes that heart sounds are caused by the opening and closing of heart valves.