I'm always a little nervous to haul out my sketching gear during a highbrow concert, like this a capella madrigal event at Vassar College. You don't want to be scratching away on a big charcoal pad, or knocking over jugs of water in some lady's lap.
So I recommend this technique, which is super-quiet and low key. You just get two brushpens, one with brown or gray and the other with black. Do a quick pencil layin, and just draw the figures as shapes rather than lines.
The fountain pen ink in the waterbrushes goes on silently. It works nicely with the all-black costumes they wear at classical concerts. And when they turn the house lights way down low you can still see what you're doing. These figures are only about two inches tall, and in the low light I could hardly see what I was doing.
No matter how carefully I hold onto the stuff in my left hand I always manage to drop a pencil under someone's seat.