You claimed someone was walking his dog around the time of the murders and heard the shots. The person who heard what he thought were shots were heard before midnight. Unless the murders took place on Tuesday the 6th before midnight he didn't hear the shots. if they did take place before midnight then Jeremy is screwed because Nevill can't have called him at 3:15am if he died hours earlier.
There is nothing though that suggests the murders were so early.
Strangely enough a lot of guilters first thought that the murders did happen at midnight,just because it said so somewhere

However,the shots that Smith heard,were earlier.I'd have said not long after Jeremy had left for home.
If a few windows at the farmhouse had been open at the time,then a crack of a gun would have been heard as it's a quiet area.
Don't forget,there were a few guns around,including a shotgun which would have been heard and the same gun had Sheila's fingerprints on it. She may have fired it out of a window for all we know,being as there was no mention of any shot found within.