This is EVP, extreme.
electronic voice phenomena (EVP) are speech or speech-like sounds, which are inaudible during recording but detected on electronic recording media upon playback.
I donít recommend this at all. When I was in high school, three of my friends did this, and their lives were permanently, altered.
Cryptic Recording, is simply, setting an audio recorder on the ground, next to a tombstone, in darkness, and leaving. Return, a few hours later, retrieve the device, and play it back.