Right phonetic
乌 — supplies the sound exactly (wū). Originally a pictograph of a crow, simplified from
烏. Faintly semantic too:
乌 alone is also onomatopoeic for a crow's caw, so
呜 reinforces the throaty 'wooo' weeping/whistling sound. Same phonetic in
钨 (tungsten),
邬 (surname). Common compounds:
呜咽 (sob),
呜呼 (alas).