What is the ruling on installing echo sound amplification devices in mosques?

Using an echo device is permissible if it only improves the sound within the mosque. However, if it echoes letters and adds to them, then it is forbidden because it alters the words of Allah. Raising the voice with the Quran via loudspeakers outside the mosque from the minaret is permissible, provided it does not harm or cause disturbance to neighboring mosques or homes. If it causes harm and disturbance, then it is not permissible, due to the Prophet's (peace be upon him) prohibition of harming worshipers by raising one's voice.