feathers used decorate coffin, large wooden fan
翣 centers on a large fan made of feathers, used decoratively in funeral rites to adorn a coffin.
翣 combines 羽 (feathers) with 妾, which likely contributes the sound. The character originally referred to a feather-decorated fan used in funeral ceremonies.
Feathers 羽 above a servant 妾 suggest a large fan held by an attendant to decorate a coffin during a funeral.
For ソウ, imagine a solemn funeral procession where a large feather fan is carried, and the sound of a deep gong echoes: so-u -> ソウ.