Causes of soft S1 include

Ref Harrison 19 th ed pg 1447 S1 becomes softer in the later stages of MS when the leaflets are rigid and calcified, after exposure to b-adrenergic receptor blockers, with long PR intervals, and with left ventricular contractile dysfunc- tion. The intensity of hea sounds, however, can be reduced by any process that increases the distance between the stethoscope and the responsible cardiac event, including mechanical ventilation, obstruc- tive lung disease, obesity, pneumothorax, and a pericardial effusion.