Final answer:
The disease of the mitral valve is best heard during the closing sounds of the heart, known as the first (lub) and second (dub) heart sounds.
Step-by-step explanation:
In a normal, healthy heart, the sounds of the closing of the mitral valve, also known as the bicuspid valve or the left atrioventricular valve, can be heard. These sounds are part of the first and second heart sounds, known as S₁ and S₂ respectively. S₁ is the sound of the closing of the atrioventricular valves during ventricular contraction and is described as a "lub" (first heart sound), while S₂ is the sound of the closing of the semilunar valves during ventricular diastole and is described as a "dub" (second heart sound).