It depends on where it is being held at, what type of event, where you sit and what not. Sometimes a symphony may hold a gala night that is basically a black tie affair, but that is usually reserved for private donors to the symphony and other high rollers.
For the most part, some slacks and a nice shirt will do, maybe a tie. If you are sitting closer to the ochestra pit in some more expensive seats then you may want to consider slacks, tie and maybe a jacket.