This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
譄 combines 言 (speech) with 曾, which likely contributes the sound. The exact historical development is uncertain.
With 言 (speech) beside 曾, imagine an ancient scribe recording words on a layered stack of bamboo slips, but the exact meaning is lost.
For ソウ, picture a soul (ソウ) whispering forgotten words through the speech radical 言: soul -> ソウ.