Back in high school, audition jitters aside, I adored being on the competitive orchestra circuit. The long rehearsals led to beautiful music and amazing friendships. And if you want to learn about a local arts group that creates music, friendships, and unity across cultural and social gaps, check out the Boston Children’s Chorus tomorrow, November 5 (6-7pm), at the Boston Public Library. The BCC -- which uses music to unify Boston’s diverse communities, inspire social change, and provide kids with music education and performance opportunities -- will preview a documentary about its recent tour to Jordan and celebrate the release of their This is the Sound of Harmony CD.