The topic of backwards-recording in music often presents itself. Especially in rock and electronic music, artists use these "back-masking" techniques and sometimes are accused of using them to subliminally influence their audience. It is very easy to record vocals or any other sounds backwards into music; artists that use computers or samplers can accomplish this with the press of a button. Vocals or other noises are simply recorded as usual and then flipped around backwards by the computer to be embedded "unintelligibly" into the music. Some argue, however, that it is still intelligible enough for your subconscious mind to pick up on it. Take, for example, this waveform of some drums and a simple guitar part (~100k .aif .wav .au). It should be noticed that the guitar notes get increasingly lower, and so they will get increasingly higher when the clip is reversed. The words "give me money" have also been reversed and mixed into this clip using a simple freeware program. They are definitely not consciously discernible when the audio is played forwards, but when it is reversed, the words will be "forwards" again and although faint, they can be made out (~100k .aif .wav .au). This is an example of back-masking, using a sample phrase (do not actually "give me money"!) that was completed in under ten minutes. This sound could now potentially influence someone subliminally. An interesting note about back-masked words is that they can easily be disguised in other sounds such as crashing waves or lapping water, blowing air or wind, cars driving by, or other ambient noises of that nature. In this clip, a note on the guitar was simply played in the background to help blend the voice into the music. This process and can be done just as easily with analog audio equipment as on a computer, and to a degree, can be mimicked in human speech. It is much harder to do, but words can indeed be back-masked phonetically as they are in "Stairway to Heaven" by Led Zeppelin. The Zeppelin Backwards! site has audio clips available to download.
There are probably thousands and possibly millions of different albums out there that contain backwards lyrics. Some bands do this openly, as in Masterhit by Front 242 on the Backwards Masking web site. Pitch Shifter, a band known for incorporating technology into their music, also includes backwards sounds in their songs and has been doing so for years. On their 1996 album Infotainment? they even go so far as to include two tracks of featured back-masked lyrics. The first track, "Introductory Disclaimer", begins with a voice explaining, essentially, that a subliminal message is about to be played. It then proceeds to play soft, waving string music in which the words "Pitch Shifter are good: Pitch Shifter are our pals" are embedded somewhat conspicuously. The second occurrence, "(Harmless) Interlude" on track six, contains the same lyrics and music. In contrast to this open display of back-masking, the band Judas Priest was taken to court for allegedly using subliminal lyrics to urge their listeners to commit suicide. Metallica, another metal band, has supposedly embedded words phonetically into their songs. This had been well documented at a web site called Acillatem, but it has unfortunately disappeared from the web. In any case, back-masking exists and humans should be aware of what all is possible in audio production.