Not really an illusion, but a case of pareidolia. Our brain is wired to recognize patterns in chaos, even the more so when prompted in advance. Another example is 'seeing faces' in everything that vaguely resembles a face.
There's a skeptoid episode about this phenomenon: http://skeptoid.com/episodes/4105, 'When People Talk Backwards'. Is has more audio examples, from the same researchers. They were using "three-tone sinusoidal replica", or a complicated sine wave sound. They found that people were able to perceive speech, when in fact there were no traditional speech sounds present in the signal. (from the skeptoid podcast transcription).
It is also similar to the well-known phenomenon of Mondegreens. My favorite being the Beatles phrase "the girl with colitis goes by" in Lucy in The Sky with Diamonds
There's a skeptoid episode about this phenomenon: http://skeptoid.com/episodes/4105, 'When People Talk Backwards'. Is has more audio examples, from the same researchers. They were using "three-tone sinusoidal replica", or a complicated sine wave sound. They found that people were able to perceive speech, when in fact there were no traditional speech sounds present in the signal. (from the skeptoid podcast transcription).