Contemporary recording techniques allow us to add novel features to music. One of the more interesting techniques is physical reversal of sound or backward masking.
Likely the first purposely reversed vocal track came from The Beatles. When George Martin was recording the song Rain in April 1966 he wanted to add something to John Lennon’s vocal. He copied a segment of John’s vocal from the original four-track tape to another recorder. He reversed the sound and recorded it back to the original tape.
This link presents some interesting background on audio reversals. I remember the Beatles White Album. I listened to the announcer say “Number Nine” on the song Revolution 9. When I played the record backwards, I was sure that I heard the announcer say “Turn me on, dead man”.