S1 is caused primarily by the :

Accentuated S1: A louder-than-normal "lub" can occur during states of high cardiac output, such as exercise, fever, or hyperthyroidism. It can also indicate mitral stenosis, where the mitral valve becomes narrowed and stiff, snapping shut with more force.Diminished S1: A faint "lub" might suggest reduced ventricular contractility or a condition like mitral regurgitation, where the valve doesn't close completely, preventing the sharp "snap" of a healthy S1.Split S1: Occasionally, the mitral and tricuspid valves do not close at exactly the same time. This results in a "split" sound, where two distinct components of the "lub" can be heard. While sometimes normal, a wide split can point to electrical conduction issues, such as a bundle branch block. Beyond the Sound: The Importance of Rhythm