Praise be to Allah.
The woman’s voice is not `Awrah in principle, for women used to complain to the Prophet (peace and blessings of Allah be upon him), and ask him about Islamic matters. They also did that with the Rightly-Guided caliphs (may Allah be pleased with them) and the rulers after them. They would also greet non-Mahram men with Salam and return greetings, and none of the Imams of Islam denounced them for that.
But it is not permissible for a woman to speak in a soft or alluring voice , because Allah says (interpretation of the meaning):
“O wives of the Prophet! You are not like any other women. If you keep your duty (to Allah), then be not soft in speech, lest he in whose heart is a disease (of hypocrisy, or evil desire for adultery) should be moved with desire, but speak in an honourable manner.” [Al-Ahzab 33:32]
This is because men may be tempted by that as is indicated by this verse.
And Allah is the source of strength. (Fatawa al-Lajnah al-Da’imah lil Buhuth al-`Ilmiyyah wal ‘Ifta’, 6/83).
And Allah knows best.