Electronic Voice Phenomena (EVP)
Also:Voices captured on audio equipment

From ectoweb.com:

EVP is the use of audio equipment to capture voices. Typically, voices appear on tape or video that should not be there. For instance, when there was no one in the room or the voices are in a foreign language. In many case, the voices seem to be responding to an investigator or a situation. No voices are heard by the people present in the recording but, after reviewing the tapes, there are strange voices recorded.

Also: Radio Voice Phenomena (RVP) - The voice of a ghost or spirit communicating through a regular radio.


From The American Association of Electronic Voice Phenomena:

EVP are anomalous, intelligible speech recorded in or produced by electronic devices, and for which no currently understood physical explanations can account.