Which heart sound is the loudest when auscultated and reason?

S1 (first heart sound) is the loudest heart sound when auscultated.

It occurs when the the mitral and tricuspid valves close at the onset of systole. The mitral valve lies between the left atrium and left ventricle, while the tricuspid valve separates the right atrium from right ventricle. These two valves come together and shut tightly, abruptly halting the flow of blood backward through them into the atria. The closure of these valves produces the first heart sound, which is the loudest because of the loud components caused by the high pressure generated in the ventricles during systole.