My friend is 6 months pregnant and it’s not unusual for me to find her sitting comfortably on her couch, a book in one hand, with massive earphones pressed against her round belly. They call it the Mozart effect, a theory suggesting that listening to Mozart early on in life–preferably at prenatal age–will result in a smarter child. The professional practice of music therapy has been around in the United States since the 1940s. Therapists use music to rehabilitate patients primarily...