As a performer, and an occasional attendee, what people wear doesn't matter. It's the music that's important. But wearing slacks and a button down seems right for the room.

That said, I'd encourage anyone to go to a concert wearing jeans vs not going to a concert. The music is what's important.

Actually, with regard to whether the sound is better, I think the biggest point that I like about live orchestras is that I can see the performers... each performance is different, I can see where different sounds are coming from and see the passion each player is putting into it. The acoustics are of course also great since the conductor is varying volume to match the room, but it really connects me to the music to watch the person playing the line I'm following with my ear.