There are so many plausible theories (and some less plausible) surrounding the sound moderator that someone could probably fill a book with them alone.
Some suggesting the extended family had a hand in it, some the police and some a collaboration between the two (for those familiar with the Avery case similar to the theory that the police asked the ex bf to move the victim's car with the spare key)
I'm firmly leaning now towards JB's guilt but if we accept that he knew how to enter and leave the locked farm house then it's surely fair to believe that other members of the family could have had this knowledge too.
The general theory is that JB pretending to be Sheila carried out the murders. You could also theorise that someone pretending to be JB, pretending to be Sheila could have carried them out. In that scenario a sleepy JB answering the phone would need to not question the male voice pretending to be NB. Too far fetched for me but let's throw it in there.
If it is true that AE and JB both wanted to some day end up living in the Grandmother's house I guess there was never going to be a happy ending for someone.