Try here :-) for a simplified explaination
The first heart sound - S1 - is in time with the pulse in your carotid artery in your neck. The sound of the tricuspid valve closing may be louder in patients with pulmonary hypertension due to increased pressure beyond the valve. Non-heart-related factors such as obesity, muscularity, emphysema, and fluid around the heart can reduce both S1 and S2.
The position of the valves when the ventricles contract can have a big effect on the first heart sound. If the valves are wide open when the ventricule contracts, a loud S1 is heard. This can occur with anemia, fever or hyperthyroid.
When the valves are partly closed when the ventricule contracts, S1 is faint. Beta-blockers produce a fainter S1. Structural changes in the heart valves can also affect S1. Fibrosis and calcification of the mitral valve may reduce S1, while stenosis of the mitral valve may cause a louder S1.
The second heart sound marks the beginning of diastole - the heart's relaxation phase - when the ventricles fill with blood. In children and teenagers, S2 may be more pronounced. Right ventricular ejection time is slightly longer than left ventricular ejection time. As a result, the pulmonic valve closes a little later than the aortic valve.
Higher closing pressures occur in patients with chronic high blood pressure, pulmonary hypertension, or during exercise or excitement. This results in a louder A2 (the closing sound of the aortic valve).
On the other hand, low blood pressure reduces the sound. The second heart sound may be "split" in patients with right bundle branch block, which results in delayed pulmonic valve closing. Left bundle branch block may cause aortic valve closing (A2) to be slower than pulmonic valve closing (P2).
During diastole there are 2 sounds of ventricular filling: The first is from the atrial walls and the second is from the contraction of the atriums. The third heart sound is caused by vibration of the ventricular walls, resulting from the first rapid filling so it is heard just after S2. The third heart sound is low in frequency and intensity. An S3 is commonly heard in children and young adults. In older adults and the elderly with heart disease, an S3 often means heart failure.
The fourth heart sound occurs during the second phase of ventricular filling: when the atriums contract just before S1. As with S3, the fourth heart sound is thought to be caused by the vibration of valves, supporting structures, and the ventricular walls. An abnormal S4 is heard in people with conditions that increase resistance to ventricular filling, such as a weak left ventricle.