Here Thomas Edison in 1877 with his invention, the phonograph. He called the reverse sounding "novel and sweet although different".
Backmasking goes as far back toward Thomas Edison, an inventor and businessman. After inventing the phonograph in 1877, he noticed that music had a reverse sounding, call it "novel and sweet but although different". In the 1950s, avant-garde musicians started involving reverse audio into their music. They ran reel-to-reel tape recorders backwards, which led to many strange melodies and lyrics. It wasn't till 1966, when the Beatles released their 7th studio album, Revolver was backward sounds were pushed into the mainstream. Such songs like Rain and Tomorrow Never Knows has incorporated backwards sounds and messages from the album. The Beatles kept using these methods throughout their albums. While recording their 8th album, Sgt. Pepper Lonely Heart Club Band, there has been rumors that Paul McCartney, one of the members of the Fab Four, supposedly died before the album was released, leading others to suspect that their backwards messages were hints about his death and that the other Paul was an imposter. Clues lead up and came up to haunt the band over the years especially when it came to their 9th studio album, The Beatles (commonly known as The White Album). In their song, I'm So Tired, speculation rose when at the end of the song, John Lennon was speaking incomprehensibly. When played in reverse, some hear John saying "Paul is dead man, miss him, miss him, miss him". Same goes for their song Revolution 9, also from the White Album, an avant-garde track that contained many weird effects and noises. The phrase repeats throughout the song 'number nine', reverse sounds like 'turn me on dead man', made everyone believe that the rumors were true even though denied by both band and record company. After the finding of the backwards messages in The Beatles albums, many more songs were found to contain backwards messages, mostly in rock music. During the 1970s, fundamentalist Christian groups started protesting and stating that the backwards messages can reach the subconscious and be accepted by the listener. They also stated that Satan himself put these messages to control and persuade the youth. Although many artists from the 70s/80s were accused of having subliminal messages (like Pink Floyd, Electric Light Orchestra, Queen, Styx, AC/DC, Judas Priest, The Eagles, The Rolling Stones, Black Oak Arkansas, Rush, etc.).