I agree. Talk to your Pastor and it may be best for him to address the issue. It was out of order

PRAY, PRAY, PRAY...and absolutely talk to your Pastor, as far as getting your Pastor involved, I would try to handle it yourself for the most part and try to get the matter resolved at the lowest level.
As far as what to say: let her know that you have been placed in this position of authority and that she needs to respect that because. If she does not respect that then she is basically disrespecting the Pastor of the Church and that cannot be tolerated. And if all else fails and she continues to be disrespectful then she has gotz to go!
I have always been taught to praise in public and rebuke in private so I would talk to her on the side and not bring the rest of the choir in on what is going on, now if she says something to prompt a response during rehearsal, then you have no other choice but to rebuke that spirit of rebellion. Let her know that her actions were highly inappropriate and at such a critical juncture in the service. Are main objectives in Ministry is to win souls to Christ, and any confusion can be a hindrance.
I am sure that your Pastor was wondering what was going on,

did he ask you about it?