I'll give you a bump just because it would suck to have this problem for 2+ months with no fix yet. I have no words of wisdom except that you might try posting in the "Support" forum to see if someone who frequents that area can help you.
Are you using Ventrillo? More specifically is this person using a 'push to talk' key? Because in another game this happened to me and I'm not sure why. If so then change the activation to a voice level and that, hopefully, should fix the problem.
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