First heart sound is soft in all, except
S1 is loud in short PR interval
"S1 is heard due to closure of AV valves"

THE INTENSITY OF S1  DEPENDS UPON -
1)   Position of mitral leaflets at the onset of ventricular systole 

Mitral leaflets at greater distance than normal - S1 loud (short PR intervar)


Mitral leaflets at a lesser distance than normal - S1 soft (long P-R intervals)

2)   The rate of rise of left ventricular pressure 

Low rate of rise of pressure
S1 soft (VSD and MR)Q
High rate of rise ofpressure
S1 loud (ASQ)

3)   Presence or absence of structural disease of mitral valve 

Calcification of mitral leafierQ → Soft S1
Defective closure in MRQ → Soft S1                              

4)   Amount of tissue, air or fluid between heart and stethoscope

Pericardial effusionQ → Soft S1
ObesityQ  →   Soft S1                             
EmphyseinaeQ →  Soft S1

Explanation of the loud S1  in short PR interval

PR interval denotes the time interval between atrial and ventricular contraction.
PR interval occurs due to delay at the A.V. node. This delay allows maximal time for atrial contraction and protects the ventricle from fast stimulation. What would be the implication of early ventricular contraction on first heart sound ??
It has been already discussed that first heart sound occurs due to closure of the mitral valve at the start of ventricular systole or contraction.
At this time increased intraventricular pressure exceeds the atrial pressure and the A.V. valves close, producing first heart sound.
Keep in mind one important fact that just before the start of the ventricular systole, the atrial contraction stops. (The atrial contraction occurs during late diastole, just before the start of ventricular systole).
With reduced PR interval the ventricle starts contracting, while the atria is still in a state of contraction.
Thus at the start of a ventricular contraction, the mitral valve leaflets are maximally separated because the atria is still contracting.
That means that ventricular systole will now push the mitral leaflet from a greater distance than usual.
This results in loud S (imagine the sound produced when a door is closed form a short distance or from a widely opened position. When the door is forced to shut from a large distance, obviously more sound will be produced).
The reverse happens in longer PR intervals -
Larger PR interval means there is more time lag between atrial and ventricular contraction.
Thus when ventricular systole begins the mitral leaflets have already begun to close due to atrial relaxation.
Thus less force is applied to the mitral valve and its closing velocity is decreased resulting in soft S1
Short PR interval
Ventricular septal defect
Mitral regurgitation
Calcified valve