Is it permissible for a woman wearing the niqab to deliver a sermon in front of men, and what is the ruling of the four schools of thought on this, especially the Maliki school?

Scholars have differed regarding the ruling on a woman's voice, whether it is 'awrah (private/concealable) or not. The four schools of thought (Hanafi, Maliki, Shafi'i, and Hanbali) agree that a woman's voice is not 'awrah in its essence. However, it is forbidden to derive pleasure from hearing it, or if one fears temptation from it, especially if it involves softness in speech, or elongation, or feminization. But if the voice is normal and free from temptation, such as for teaching, seeking a fatwa, asking a question, or consultation, there is no harm in it. This is what Al-Ghazali and Sayyid Qutb, may Allah have mercy on them, stipulated. In summary: it is permissible for a woman to give lessons in the presence of non-mahram men if her voice is known, without softness or obscenity, and if there is a need for it. However, it is preferable to avoid it if she is not obliged to do so, in order to avert suspicion.