window
牕 centers on an opening that lets in light and air: a window. The other given readings (てんまど, けむだし) are specific types of windows or smoke vents, which are natural applications of the core window idea.
牕 is a phono-semantic compound: 片 (split wood, here suggesting a wooden frame or panel) combined with 悤, which likely contributes the sound. The character is a variant form of 窗, which also means window.
The left side 片 looks like a wooden frame or panel, and the right side 悤 contains 心 (heart) under a roof-like shape. Picture a wooden-framed window that lets light into the heart of a room.
For ソウ, imagine the soft sound of wind blowing through an open window: 'sou' (like the English word 'sow' but with a long 'o') — the gentle breeze enters through the 牕.