The US Navy also investigated this issue as the shooting took place at their base and the paper was publicly available as well.
“On September 25, 2013, the Secretary of the Navy appointed Admiral John Richardson, USN, to conduct an in-depth investigation into the full range of security, contractor, personnel, and other factors related to the September 16, 2013 incident at the WNY. The investigative effort was organized into five areas:”
They continued to elaborate these five areas followingly and it started by tracking a personal history of the shooter in and out of the military as he previously served as a Sailor in the US Navy. Then, they proceeded to investigate why he got a security clearance to access the base and how the Navy responded to the mass shooting.
Their finding in this paper is more detailed than in the report from the oversight committee and there is another angle of information that he asked his supervisor of the voices. “Alexis asked her, “Can’t you hear that?” The second shift deployment supervisor said she did not hear anything and told Alexis so.” This looks like an intel from the interview with this supervisor by the Navy investigators.
This information means that the voices were not heard from the outside but just inside his brain. That is why he still heard, but she could not. At least, we can exclude the possibility that he heard the sound through the walls, ceilings and floors.
Although, it cannot still deny a possibility that he was hallucinated. This is always the hardest part of this electromagnetic manipulation that it is quite hard to trace as it is invisible in nature.
Just in case, she might have heard something in her head, when her brain was coded similarly to his. I have no idea of this probability, but it should not be high in my experience as almost all of them have not heard what I was hearing, but I saw some hearing the same in several occasions.