Electronic Voice Phenomena (EVP)
For decades, paranormal believers have been picking up stray voices on tape recordings, which some think come from dead people or extraterrestrials. As a website for paranormal enthusiasts explains, picking up electronic voice phenomena, or EVP, often requires a person to listen to the recording with headphones and work hard to pick the voices out from the background noise, and sometimes a recording will have to be played in reverse for the message to appear [source: ATC]. (A similar technique was used by teenage Beatles conspiracy theorists in the late '60s, when they played "Strawberry Fields Forever" backward in an effort to hear what sounded to some like "I buried Paul.")
But not everybody believes EVP is truly paranormal. The Skeptic's Dictionary, for example, offers effects such as interference from a local CB operator, and various sorts of naturally occurring electronic distortion as possible explanations.