This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development is uncertain. The modern form visually contains 木 and 雙, but the original meaning and component function are unclear.
The left side is a tree 木, and the right side is 雙, which suggests a pair. Picture a pair of trees, but the overall meaning remains unclear.
For ソウ, imagine a pair of trees swaying in the wind: the sound of the wind through the leaves is like a soft 'sou' -> ソウ.