Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
If it is needed to follow an imam behind walls through audio speakers because of lack of space or for necessity, then it should be fine.
Answering your question, Sheikh Kifah Mustapha, the Imam and Director of The Prayer Center of Orland Park and representative for Dar El Fatwa of Lebanon in the US, states:
Yes it is permissible to follow the imam through loudspeakers behind the walls. We have a rule in the Shari`ah that says: “if a matter got limited (in access), it shall be expanded.”
This rule is based on the verse of the Quran where Allah the Almighty says: {and strive for Allah in the striving due manner for Him. He has chosen you and He has not placed upon you in the religion any difficulty.} (Al-Hajj 22:77)
From this verse and rule if it is needed to follow an imam behind walls through audio speakers because of lack of space or for necessity, then it should be fine.
Almighty Allah knows best.