There have been many rumors and speculations that famous songs actually contain subliminal messages. Subliminal messages used in songs are verbal or aural in natured, and are delivered either very quickly or back at such low volumes that the audience is unaware of receiving it. Such charges are made not just to rock music, but even Christian songs as well!
A famous subliminal message in music would be the song Stairway to Heaven by Led Zeppelin, that apparently when played backwards contains satanic messages. This is a form of reverse subliminal messages. More often than not, these are inserted for entertainment purposes or to increase sales of the album. It has been speculated that Led Zeppelin inserted the message as they thought that if people played the record back enough times, it would spoil it and hence lead to increased sales for them!
Other famous subliminal messages in music include one involving The Beatles, specifically the song titled Come Together, which is said to be about sex with John Lennon. Even Britney Spears, in the song "I'm not that Innocent" had supposedly inserted the message "Sleep with me I'm not too young" when played backwards! The rock group Queen's song "Another One Bites the Dust" is also purported to be talking about drug use when played backwards.
Several years ago, there was a court case involving the suing of a rock group "Judas Priest" which had inserted the subliminal message of "Do it" in their song, which the parents felt induced their children to commit suicide.
Before getting carried away and panicking about the various songs that you have listened to before, the real issue is not whether there are actual messages included in songs, but if these have any influence on the audience. If you were to hear such a message played backwards, would you be able to understand it, or subconsciously alter your behavior?
Research has shown that when participants listened to messages recorded backwards, they are unable to decode what it actually was when played normally. Their behavior was not altered in any noticeable way. After all, if listening to metal bands that proclaim death and gore do not injustice people to go on a murder rampage, it is inadvertently that such subliminal messages in music can have significant adverse effects on the audience.