One of the big features in Whispers In the Dark is electronic voice phenomena. This is when voices or other sounds are recorded which were not audible to the people present. Sometimes the words or phrases seem random and other times they seem to respond to the people at the site. It's a common investigative tool for paranormal investigators.
EVPs are not without controversy. Humans are very good at picking up patterns, to the point that we will see patterns even when they're not really there. For example, ever seen a face in an electrical socket?
Intellectually, we know it's not a face. But any image with two objects above and one below will resolve into a face for most people. We do the same thing with random noises, which is thought to be the origins for mermaids singing at sea: the random noise of the waves resolved into song for bored sailors. Good paranormal investigators recognize this difficulty and try to take it into account when deciding if something is an EVP or simply random background static.
The idea of being able to communicate after death is probably one of the most intriguing mysteries to the human mind and one we've probably been searching for since the cave days. Today, most investigators use handheld digital recorders to capture EVPs, although they are vulnerable to radio interference. It's the easiest option for an amateur ghost hunter to try but one of the most difficult ones to verify.