A majority of most orchestral audiences probably don’t even realize that the tuxedo-clad musicians in front of them are affiliated with organized labor, but American orchestras have been union shops since the turn of the 20th century. Well, most of them. The Boston Symphony, one of America’s top bands, furiously resisted all organizing efforts for nearly half a century before finally embracing the union in 1942. Since then, Boston’s musicians have become some of the most influential in the country.