backmasking

:  the encoding of audio materials (such as words conveying a secret message) on a recording in such a way that they can only be heard and understood when the recording is played backwards Of the new bills, Rhode Island's is unique in that it calls for several different labels, including one that says “parental warning: this record contains backmasking that makes a verbal statement when this program is played backwards” … — Richard Harrington, Washington Post, 14 Mar. 1990 —called also backward masking