gather, flow into (water), sound of waters flowing together
潨 centers on water coming together: streams gathering, flowing into one another, and the sound they make when they meet.
潨 combines 氵 (water) with 衆, which likely contributes the sound and the idea of many things coming together, as in a crowd of waters.
Water 氵 on the left and 衆 (crowd) on the right: imagine many streams gathering like a crowd of people, their waters flowing together with a rushing sound.
For ソウ, picture the sound of waters flowing together as a soft 'sō' whisper: sō -> ソウ, like the murmur of converging streams.