burnblue @ Flickr
When two sounds with two different frequencies are played in stereo (i.e. to each ear), binaural beats are heard. These aren't real but equal to the difference between the stereo frequencies. You can listen to one here (headphones needed, toggle between each ear to see the effect vanish).
There are several companies that claim such binaural beats improve mental functioning and memory by reverse engineering brain frequencies via the audio. Naturally, the combined simplicity and complexity of this idea caught certain people's imagination, to the point that a musician-physicist mate of mine wondered it there was anything to it last year during exams.
There isn't. In this recent study binaural beats had zero effect on brain frequencies and, what is more, participants in the experimental group were more forgetful and more depressed than controls.
Cognitive neuroscience, one. New Age monkeys, nil.
(If you want to see more on music and mind, see this post.)