If you hear voices that aren't really there, are you "mad"? Maybe - auditory hallucinations, most commonly voices, are one of the characteristic symptoms of schizophrenia and are sometimes also a feature of severe depression and mania. (For more on hallucinations, try this paper. It's long, but it's worth a read because it's one of the few academic papers containing phrases like "Do YOU want a slap in the head?" and "Die, bitch".) But anyone can find themselves hearing things that others don't - it just takes a little prompting.
Via the excellent Mind Hacks, I came across this great little page by Cambridge neuroscientist Matt Davis, which gives some examples of Sine Wave Speech. Essentially, Sine Wave Speech is a voice recording which has been digitally degraded so that it's little more than a collection of beeps and bleeps (sine waves, in fact). When you first listen to it, it sounds something like R2D2 on ecstasy.
However, if you listen to the original, unaltered voice and then go back to the sine-wave version, you can hear the words - the bleeps suddenly sound like a voice. The change is striking - so much so that if you didn't know what was going on, you'd probably think you were listening to a whole new clip. This effect is one example of the brain's tendency to perceive a pattern in a stream of noise, when an expected pattern is known. We're all familiar with this in the realm of vision - if you're looking for something amongst a pile of clutter, your eye is drawn to it, but you won't even register all the other rubbish that's around it. We're not used to the same thing happening with sound, though, I suppose because we generally hear things without much difficulty.
Sine Wave Speech really is a form of voice, but the same principle can lead people to hear voices in the unlikeliest places. Hence the recent case of the Amazing Satanic Islamic Doll, also known by its Jihad name, "Little Mommy Cuddle 'n Coo". This is a kid's toy made by Fisher Price which, when hugged, plays recorded baby noises. A month or so ago a concerned parent somewhere in America decided that one of the recorded baby noises sounded like a voice with a disturbing message. Have a listen.
Did you hear any speech the first time you heard the doll's noise? Probably not, but I'd bet that after you'd heard the claim that the doll was saying "Islam is the Light", the same noise started to sound rather different. After several listens, I now can't hear the noise as anything other than a voice saying that phrase, even though intellectually I know that this is absurd (if Fisher Price were going to plant a message like that they'd be more likely to use "Buy more toys").
The doll is a post-9/11 rerun of the infamous Satanic backmasking scare. If you a play a record backwards, you'll hear various distorted sounds, some of which could be interpreted as speech, if that's what you're expecting to hear. During the 1980s, some concerned citizens (again in America) thought that they could hear Satanic or sexual messages hidden in heavy metal records when they were played backwards. Upon hearing this, several bands liked the idea and actually did put backwards messages in future songs, but most of the allegations were based on people hearing voices in noise, because that's what they expected to hear.
One man believes that the problem goes far beyond metal music and that everyone has hidden backwards messages in everything they say, messages which reveal their subconscious thoughts. Cripes. He gives a lot of examples here. Note that he helpfully tells you exactly what to expect to hear before you've heard to recordings.
Anyway, in original video above, a concerned mom argues that the Muslim message must be real because she can't believe that people across the country are "all hearing things". She doesn't understand that you don't have to be hallucinating to over-interpret noise. It happens to the best of us, and once you know what to expect you really hear the voice, clear as day, whether or not you "believe in it" or "want to hear it". It's not simply the power of suggestion.
On the other hand some psychologists have claimed that suggestion alone can make people hear things. This is generally called the "White Christmas Effect". The original experiment in support of this claim took 78 female students who were told - "in a firm and serious tone of voice" -
I want you to close your eyes and to hear a phonograph record with words and music playing White Christmas. Keep listening to the phonograph record playing White Christmas until I tell you to stop.
As you may have guessed from the language used in sentence, this experiment took place in 1964. Anyway, the students were ordered to hear White Christmas playing in a silent room, and then asked to check a box indicating whether they in fact heard it. 5% said that "I heard the phonograph record of White Christmas clearly and believed that the record was actually playing" while another 49% said "I heard the phonograph record of White Christmas clearly but knew there was no record actually playing."
So according to this experiment, over half of 1960s female students actually "heard" a record playing, simply because someone told them to. (In another part of the experiment, many of them "saw" a cat.) It's very hard to know what to make of this, because it's obvious that some of them may have just been saying that they heard the music because they thought this is what was expected of them. Bear in mind as well that the lead experimenter, Theodore Xenophon Barber, later became interested in some rather dubious stuff. Even so, the paper became popular, and led to a small industry of music-based hallucination research.
More modern work inspired by the White Christmas experiment has been a bit more rigorous. In 2001, two Dutch psychologists took 47 students and told them to listen to some white noise. The students were told that the song White Christmas might be played quietly at some points, and they should press a button whenever they heard it. 14 of them pressed the button at least once (on average, three times each), although the song was never actually played. This implies that a substantial number of healthy people will hear something simply because they expect to hear it, even if what they are actually listening to sounds nothing like it.
The lesson of all of this, if you need it spelled out, is that your eyes and ears are not windows through which you have direct access to reality. Your brain is actively constructing your perceptions of the world based on prior knowledge as well as sense data. But I feel like I'm getting dangerously close to talking philosophy here, so I'd better quit while I'm ahead.