What is EVP

Cryptozoology (from Greek κρυπτός, kryptos, "hidden" + zoology; literally, "study of EVP - or electronic voice phenomena - is a mysterious event in which human-sounding voices from an unknown source are heard on recording tape, in radio station noise and other electronic media. Most often, EVPs have been captured on audiotape. The mysterious voices are not heard at the time of recording; it is only when the tape is played back that the voices are heard. Sometimes amplification and noise filtering is required to hear the voices. Some EVP is more easily heard and understood than others. And they vary in gender (men and women), age (women and children), tone and emotion. They usually speak in single-words, phrases and short sentences. Sometimes they are just grunts, groans, growling and other vocal noises. EVP has been recorded speaking in various languages.

http://paranormal.about.com/cs/ghosthunting/f/blfaqs_ghosts07.htm