Is the prayer of those outside the mosque valid, if they follow the Imam via loudspeakers, when the prayer hall is full and the doors are closed?

Zayd ibn Thabit's (may Allah be pleased with him) hadith indicates the permissibility for a follower to pray behind an imam even if the imam is in a chamber unseen by the follower, or if one of them is on the roof and the other downstairs, as long as both are within the mosque. The crucial factor is the possibility of following.

A barrier between the imam and the followers does not prevent the validity of the prayer and the following. A condition for the validity of following is the follower's knowledge of the imam's movements.

If one of them is outside the mosque and sees the imam or the followers, their prayer is valid even if the rows are not connected, because the invalidating factor is absent, and the condition for validity, which is sight and the possibility of following, is present.

The connection of rows is determined by common practice and is not fixed by a specific measure, because what prevents following is what prevents sight or hearing the voice. Al-Nawawi stipulated that the distance should not be long for locations outside the mosque, and this is the view of the majority.