When you are shooting a lit stage in a dark theater your camcorder will often adjust to the lighting in the room as a whole, not the lighting on the stage.
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Get to the event early and try a few different pre-set lighting modes in your camcorder.
One that has always worked well for me has been "Sand and Snow". The mode is an unlikely choice because it is designed for very bright lighting conditions, and you are shooting in a very dimly lit place. "Sand and Snow" always gives me much better results for stage performances than I get from any other mode on my camcorder.
Give it a try!