I know I'm replying to an ancient question here, but as somebody who does a lot of this, I wanted to throw in my perspective anyway for anybody who might be reading this looking for advice.
The short answer is that you can do it on a crop body, but you'll get a lot more keepers with a full-frame body because you can crank the ISO higher and still get usable shots (albeit at a cost in terms of having to use a bigger, heavier lens to get the same f-stop on a larger sensor).
If you're on a crop body, unless you have a fairly fast lens, you're likely to be in a world of hurt. Stage lighting isn't really particularly bright. It just looks bright because you're in the dark. With an f/4 lens, you should expect to choose between a slow shutter speed and four-digit ISO levels.
For example, on the stage at the university where I regularly do this sort of thing, with a full-frame camera shooting 35mm shots at ISO 400 and f/4, the shutter speed is about 1/40th of a second, which is okay for a wind band concert if you don't care about the conductor's arm being a blur, less so for an orchestra concert, and probably useless for a percussion ensemble concert. Stopping motion much more completely at 1/250th of a second requires ISO 2500, which is fine on a full-frame, but probably dicey on a crop.
For closeups, with my 70–300L and no teleconverter, getting 1/250th means shooting at ISO 10,000+ even at f/5, which is barely even possible on a crop. Out near the end of the range where it gets into f/5.6 territory, you'd be at ISO 12,800 just to get 1/100th of a second.
I used to use a crop body on that same stage. I have full-stage shots from back then that are excessively soft because at ISO 800, I sometimes had to shoot as slow as 1/25th of a second at f/4.
Regarding focal length for a zoom, I would tend to say "longer is better". Unless the slope of the hall makes the photos look awkward from the back of the room, you should try to sit there. That way, your mirror slapping is less likely to disturb people (who will tend to sit closer).
I never need anything wider than my 24–105 for capturing the entire stage even if I'm in the middle of the audience. At the back, I'm not even near the wide end of that. On a crop body, I had a 10–22, but realistically I could have gotten away with my 17–85 and moving a little further back. I just preferred the sharpness I got from the 10–22 in the middle of its range over the 17–85 at the widest, most distorted end of its range.
And for close-ups, I sometimes end up putting a third-party 1.4x teleconverter on my 70–300L (on a full-frame body), depending on the hall.
Obviously your mileage may vary. There's a wide range of lighting levels, depending on the hall. I feel like ours is on the dimmer side of the halls that I've seen, which is nice for the performers, but bad for photographers. I've seen other halls that are considerably brighter. But if I had it all to do over again, I'd probably have bought a full-frame camera for my first DSLR and skipped the entire crop body line in spite of the much higher cost. Particularly for photos in our hall, it has made a world of difference.