The woman’s voice is not ‘awrah in principle, for women used to complain to the Prophet (peace and blessings of Allaah be upon him), and ask him about Islamic matters. They also did that with the Rightly-Guided Khaleefahs (may Allaah be pleased with them) and the rulers after them. And they would greet non-mahram men with salaams 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 Allaah says (interpretation of the meaning): 

“O wives of the Prophet! You are not like any other women. If you keep your duty (to Allaah), 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-Ahzaab 33:32] 

because men may be tempted by that, as is indicated by this aayah. And Allaah is the source of strength. 

From Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah wa’l-Iftaa, 6/83. 

Please also refer to Question no. 1121, where you will find limits and guidelines on speaking to non-mahram women. This is an important topic.