itch
瘙 centers on the sensation of itchiness, often associated with skin conditions or irritation.
瘙 combines 疒 (sickness) with 蚤, which likely contributes the sound and may relate to the idea of a crawling or irritating sensation like a flea, though the exact historical development is uncertain.
The sickness enclosure 疒 wraps around 蚤 (flea). Picture a flea bite under the skin causing an unbearable itch.
For ソウ, imagine scratching an itch so intensely that you make a 'sow' sound of relief: sow -> ソウ, as the itch is finally soothed.