Ultra-Orthodox Jews are not allowed to even listen to a women singing. What about Christians & Muslims?

first of all it's not ultra-orthodox Jews who believe this, it is orthodox Jews period. The voice of a woman singing is considered to evoke ual stimulation either consciously or sub-consciously. It is not considered adultery. The hebrew words are that the voice of a woman is עֶרְוָה which is "unchastely" or "lewdness".