religiously (islamically), she should speak in rude manner (heavy voice) in front of non-halal man.
Actually, you are highly mistaken. No person should talk in a rude manner and it is disliked to do so. The recommendation is to talk in a serious tone and not in a manner that brings forth unnecessary attention.
If she has naturally sweet voice, and she has no wrong intention, and people feel uncomfortable if she talks in dry and rude manner, then she should not change her voice/tone. but if she tries to make it appealing to men or if her intention is to attract people in a wrong way .. then its wrong in my book.
a woman knows the exact moment when she changes the tone of her voice to sound more pleasant(than what it is actually), when talking to a na mahram man.