Firstly. With the rifle against her neck/chin it will muffle the sound of the shot.
Secondly. A sound moderator on or off makes 'very little difference' to the human ear at least that is.
"The reconstruction was held on an Army range after the court took a 28-mile drive in a police escorted convoy so that firearms expert Malcolm Fletcher could demonstrate the gun. The Chelmsford Crown Court jury of seven men and five women had asked to hear the sound of shots fired from the semi-automatic weapon. Mr Fletcher fired the un-silenced rifle five times into a huge block of soap which absorbed the impact of the bullets. Then he fired four times with the silencer in place. There appeared to be very little difference."
This was obviously an issue for the jury.
And maybe the jury asked to hear the sound of shots because they could not understand why no one had heard or reported hearing any of the alleged Sheila suicide gun shots.
The jury having then specially heard the noise of firing for themselves were sufficiently helped for 10 members of the jury to make up their mind that Sheila had been murdered and could not have been alive when people first arrived at WHF on the morning of 7th August 1985.
Addition - And if witnesses including Jeremy has reported hearing gunshots in their witness statements or in testimony at trial then these reported hearings of gunshots would have been classed as evidence and not hearsay.