This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
焋 combines 火 (fire) with 壯, which likely contributes sound. The exact historical development and original meaning are uncertain.
The top part 壯 suggests strength, and the bottom 火 is fire. Picture a strong, vigorous fire, though the kanji's actual meaning is unclear.
For ソウ, imagine a strong fire that makes a loud 'sow' sound as it burns: sow -> ソウ.
—