My Panasonic HC-V700 has a 21x optical zoom but most mid range camcorders have similar specs. I also hsve a so called Intelligent Zoom up to 46x. This gives a zoom with no loss of picture quality.
Now for the sound. The camera will hear what it hears and if it is at the back of the hall the sound will be mixed with echo, people coughing etc. So using the camera mics is a non starter. What a number of us have done is to record the concert on a audio device, a decent recorder or, in my case, a Zoom H2. We then sync the audio with the video in an editor. I use Corel VideoStudio X6 but most editors will allow you to align audio with video.
I'm looking for a camcorder which can record sound (and video, of course) from a concert without distorting it. I would also like it to have a long zoom, at least 20x optical zoom. Standard definition video would do.