The murmur of MS happens when there is a pressure difference between LA and LV and both are in diastole…hence the flow (causing the murmur) through Mitral Valve occurs when LA is not contracting…with increasing severity of MS the LA pressure increases proportionately…hence in more severe MS a greater pressure difference exists between LA and LV…as a result the Mitral valve opens earlier to S2 and blood flow occurs through it for a longer time owing to the increased pressure gradient…duration is imp as the sending chamber(LA) is not actively contracting so there is no chance of increase in intensity of murmur…imagine liquid flow between chambers only becoz of pressure gradient and not any active force of contraction and you would understand why duration is the sole reflector of severity…
Loud S1 occurs in moderate MS as well…but there are many other factors that determine loudness of S1…a severely thickened calcified MS would have soft S1… MS if associated with MR may have soft S1…MS which is frequently associated with AF will produce variable S1…
Although loud P2 indicates PH due to MS and can occur only in severe MS, but P2 is per se not an attribute of mitral valve…imagine a situation where the MS is severe but has not yet caused PH, in that case it will not be a reflector of severity…any finding which is attributable to the MV itself and can be appreciated in the Mitral area is usually dedicated to dictate the severity of any MV lesion…