1. How did Dahmer manage to get his whale song tape played over the PA system so the other inmates could hear it and complain?
2. Why would both the inmates and even the guard be so stupid as to think whale song = someone screaming?
1. I think he just had the volume on his tape player turned up really loud.
2. They knew he was a cannibal serial killer, so anything he did was looked at in the worst possible light.
It wasn't played over the prison PA.
Due to his fan donations, he managed to buy a tape of whale sounds and a tape player, and was playing it at a loud volume in his small, isolated block of cells. It annoyed his neighbors, and they complained.
Whales do sound somewhat like people moaning or even sometimes screaming, when they get high-pitched. If his neighbors, and especially the guards who were aware of his crimes and already had a negative view of him heard this, i could see them mistaking him listening to these whale sounds as listening to human moaning/screaming.