Most of us remember the joyful song about teaching the world to sing, in perfect harmony. It was later used as an incredibly popular Coke commercial.  Some of you may have seen the contestant on American idol with a serious stutter, that disappeared when he sang.Both are testaments to the power of song, and of singing with others. In fact, over 40 million Americans participate in some kind of group singing. But why? What's the appeal and what’s the impact?Bestselling author ;and commentator Stacy Horn takes a look at his in Imperfect Harmony: Finding Happiness Singing with OthersMy conversation with Stacy Horn: