哞 is a pure onomatopoeia, one of many Chinese animal-sound words. Cows say 哞哞 in Chinese, just as they say 'moo' in English - a rare case of cross-language onomatopoeic similarity.
口 depicts an open mouth. As radical it marks 哞 as a sound-effect character - the mouth radical signals onomatopoeia, joining 哈呼嗒嘿, characters whose job is to render audible noises in writing.
牟 supplies the sound (móu shifted to mōu) and a direct semantic boost - 牟 itself originally meant the lowing of cattle, with 牛 cow on top and a vapor mark below. Here it doubles down: 哞 is mouth + cattle-bellow, the sound a cow makes.
