All perfect praise be to Allah, Lord of the Worlds and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
In order for following an Imam from inside or outside the mosque to be valid, Muslim jurists have stipulated that the praying person/s should be aware of the Imam`s transitions through either hearing the Takbeer, seeing the Imam or those praying behind him.
Imam Nawawi said: " Awareness of an Imam`s positions is a condition for the validity of following him. This is whether the Imam and the person following him are in the same mosque or one is there while the other isn`t, and this is according to scholarly consensus. Fellow scholars {Shafie} have said that this awareness can be achieved by hearing the Imam or those praying behind him or by seeing his positions or those of the persons following him." {Al-Majmou`, vol.4/pp.201}. Kindly review the citation of this consensus in {Al-Mawso`ah Al-Fiqhia, vol.6/pp.26}.
If the women couldn`t hear the Imam because the speakers in their Musala (Place of prayer) have malfunctioned, and thus couldn`t tell his transitions, then following him isn`t valid. In this situation, they are allowed to have their own congregation by choosing one of them to lead the prayer, even if that coincided with the men`s congregation. This is because it is disliked to make a second congregation when it is possible to follow the first, which isn`t the case here.
Moreover, it is worth pointing out that if the above situation took place during Friday prayer, then those women should pray four Raka`hs (Zuhr). And Allah Knows Best.
