AHh, the Symphony. If it's a matinee, general, a collared polo-style shirt & slack work, for evening, shirt & tie affair (As someone said earlier, no jacket needed, and only a tux for like the "gala"; I'll wear a jacket for some of the more "eventful" performances, like the first/last of the season. Women are much easier, general "work casual" attire works for them; stay away from shorts/non-dress pants/skirts & t-shirts, and they are golden.
I really enjoy going to the "symphony in the park", where it's a t-shirt & shorts 🙂