rice-steaming pot
甑 centers on a traditional pot used for steaming rice, a specific cooking vessel.
甑 combines 瓦 (earthenware) with 曾, which likely contributes the sound, to denote a clay steaming pot.
The left side 曾 looks like a layered steamer basket, and the right side 瓦 is earthenware. Together they form a clay pot for steaming rice.
For ソウ, imagine the sound of steam hissing out of the pot: 'sou' (like 'so' in 'soul') — the steam whispers 'sou' as the rice cooks.