EVP or ELECTRONIC VOICE PHENOMENA is very popular area of Paranormal Research. Basically, voices found on static or background noise.
EVP cannot be heard at the moment when the evidence is recorded. It can only be heard through electronic playback. Many skeptics say that EVP is only the result of the tendency of the human brain to recognize patterns in random stimuli known as auditory pareidolia. Paranormal grounds for the origin of EVP include living humans imprinting thoughts directly on an electronic medium through psychokinesis and communication by discarnate entities such as spirits, natural energies, beings from other dimensions, or aliens.
However there is a theory that is said to move way past the technical glitches of electronic devices. The normal person in a very quite room tend to hear a ringing sound, like static called white noise. There are some people that tend to go way past the boundaries of the human sense of hearing. They are ultra sensitive to sound that they could hear beyond the white noise and could hear a different frequency where the voices of spirits could be heard.
It is said that anyone who is hearing white noise can try to concentrate in order to hear what is beyond the sound of silence.