How does music effect the mind?

Sub Questions:
    -How does music have the ability to change your mood?
    -Why do stores play certain music?
    -How can music change the way you think/feel about something?
    -Is it possibly to become mad because of rap music?  
    -Can country type music really calm you down or is it all in your mind?
    -Does music have the ability to change your sleep cycles?
    -Is there a secret in the lyrics to change your mood or is it the way the artist is singing it?

What I already know and would like to find out:
I do not know to much about this topic yet, but this is what I do know.  I know that if you play certain raps backwards there are still words and it is like a whole different song.  Also in stores they typically play loud, fast music. I would like to learn more of the phycology behind that.  I would also like to learn things to answer all of my sub questions.