A good place to start understanding the effectiveness of Brainwave entrainement is in the following Wiki article, .
What is entrainment? http://en.wikipedia.org/wiki/Brainwave_entrainment
The following is a meta study of the effectiveness of entrainment. Please read and comment. Thanks.
http://www.transparentcorp.com/downloads/Huang-PsychologicalEffectsBrainwaveEntrainment.pdf