She may have been stunned facing the hale of bullets and was trying to hold in the pain. You have no firm basis whatsoever for your train of thought that June was wailing loud enough to alert other members of the household.
Never seen anyone scream when shot in films. Man or woman. If still alive they are usually in shock, in terrible pain and thinking about what to do next.
June would not have screamed while being shot. She was asleep. While Bamber was downstairs June woke from the pain and crawled a few feet. In shock, extreme pain and with no strenght, not sure she would start screaming.
But appreciate Sheila waking from the downstairs fight is very unlikely, so it has to be scream from a woman in another room shot 5 times.
Some supporters believe Sheila waking & getting out of bed mid massacre would somehow be a problem for Bamber.