This is an example of a diastolic decrescendo murmur occurring after an opening snap. It is associated with mitral stenosis.
The first heart sound is increased in intensity while the second heart sound is normal. An opening snap is present after the second heart sound. The low pitched rumbling murmur starts after the opening snap and lasts until mid diastole.
On the anatomy video you can see an enlarged left atrium and thickened but mobile mitral leaflets.
Turbulent blood flow from the left atrium into the left ventricle is responsible for the murmur.