First heart sound corresponds to:

The first heart sound (S1) includes mitral and tricuspid valve closure.  The First heart sound is associated with closure of the A-V Valves. The Second heart sound Is associated with closure of the Aortic and Pulmonary Valves. The Third heart sound occurs at the beginning of the middle third of diastole. Occasionally a weak, rumbling third heart sound is heard at the beginning of the middle third of diastole.   Atrial contraction sound (Fourth heart sound).  Reference: : Guyton and Hall Physiology page no  283,284