hey, thats great to hear you are headed to the symphony!
now, as someone who goes to events at lincoln center or carnegie hall at least 4 times a week, i can say that you will find a WIDE VARIETY of dress for such events.
first, you are so lucky to be seeing anne sophie-mutter in performance--that will surely be a treat!
two weeks ago at the opera i was in the same row as a girl in a TRACK SUIT! my god! now people are rarely THAT casual, but you should wear what you will be comfortable wearing! some days i go in jeans and a sweater...some days i go straight after work in a suit, so i have worn both, but more often i am fairly casual because i feel comfortable and thats all that matters.
stay away from the track suits though!