the corpse of a bird or fowl, fat
膆 centers on the physical substance of a dead bird or fowl, specifically its corpse, and by extension fat or flesh. The two meanings are linked through the idea of animal tissue.
膆 combines the flesh radical ⺼ with 素, which likely provides the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to bodily substance.
The flesh radical ⺼ on the left hints at meat or body tissue. The right side 素 (element, plain) can be thought of as the raw material. Together, picture the plain flesh of a bird's corpse, or the fat that remains.
For ソ, imagine a bird's corpse lying on the ground, and a single 'so' sound escapes from it: so -> ソ. The soft, plain sound matches the plainness of the raw flesh.