You're press, they should let you use flash. But if they don't, relax, auditoriums are not bad. And they're singing, so slow shutter speeds will work, unlike sports. You will be fine with the IS. My only concern would be white balance, as some auditoriums have terrible quality of light. My suggestion is leave it on auto white balance and do any cleanup in post. If they have 60hz lamps nothing you can do will guarantee the right colors anyways.