A mirror symbolizes how we appear to the world, or more specifically, to our peer, or social group.
That there is a face in your back, suggests you feel that someone, or a specific female, is talking about you behind your back.
That she doesn't speak, or only speaks static, suggests you don't want to hear what is being said. But that it is a recurring dream, means that it does bother you on an emotional, or subconscious level.
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Do you feel as though you are being singled out at work, home, or somewhere else you often go?
Do you feel as though someone is trying to discredit you?
Do you feel like you are constantly defending yourself, or your actions?
Do you know someone who you feel is 'pretending' to be your friend?

Hope this gives you some insight. Best Wishes!