If you can, use a spot meter. Often the musicians are much more brightly lit than their backdrops, etc.
Tripod if you can. This helps with using longer lenses at longer shutter speeds. With rapid hand movements and such, you might wish to find an ideal shutter speed for both freezing action as well as capturing motion if you wish. Then you can work out what type of film speed you need, based on the maximum aperture of the lens.
I've shot stage performances using a Mamiya 645, a 300mm lens, a 2X tele-extender, and Delta 3200. The 2x extender made my lens effectively an f/11 max aperture. I pushed my film to 6400 and worked with slightly thin negatives. My shutter speeds were in the 1/8s to 1/30s depending on where on the stage they were located.
After I get home today I'll look up a couple of the scans to show you what I was able to do from the back row of the auditorium.