Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Page 58
Question: If a woman prays behind the men and the imaam errs in his recitation of the Qur.aan and no-one corrects him, is the woman allowed to correct him if she knows?
Response: If this occurs during [the recitation of Soorah] al-Faatihah, then it is obligatory upon her to correct him, because if he errs in [the recitation of Soorah] al-Faatihah then his prayer is incorrect. If however it was in other than [the recitation of Soorah] al-Faatihah and the men are from those who are known to her and there would not be any cause for surprise if she spoke in front of them, then there is no harm in her speaking to correct the imaam. This is because the voice of the woman is not an ‘awrah [that has been legislated to protect and cover], and the evidence for this is His (Subhaanahu wa Ta’aala) saying:
So the prohibition of being soft in speech is evidence as to the permissibility of the speech [of the woman].