sip, slurp, suck
歃 centers on taking in a small amount of liquid through the mouth: sipping, slurping, or sucking.
歃 combines 臿 (likely a phonetic component) with 欠 (open mouth), suggesting an action involving the mouth. The exact historical development is uncertain.
The left side 臿 looks like a mortar and pestle, and the right side 欠 is a person with an open mouth. Imagine someone leaning over a mortar to sip or slurp the liquid inside.
For ソウ, picture someone sipping soup loudly: the sound 'soup' is close to ソウ, and the open mouth 欠 is ready to slurp.