A. A heart murmur is a sound that is created by inadequate blood flow through the heart and its large vessels, for example the aorta. Some are born with a heart murmur and further testing doesn't reveal any significant problem.This is called a physiological murmur.eart murmur Cardiology An auscultatory sound of cardiac or vascular origin, usually caused by an abnormal flow of blood in the heart due to structural defects of the valves or septum; murmurs may be benign or pathological.