glue, gum, resin, rubber, sound, shin bone
胶 centers on sticky, binding substances: glue, gum, resin, and rubber. The meanings "sound" and "shin bone" are unrelated outliers, likely from separate etymological roots or rare uses; the main cluster is about adhesives.
胶 combines ⺼ (flesh) with 交, which likely contributes the sound. The character originally referred to a type of glue made from animal parts, hence the flesh component. The meanings "sound" and "shin bone" may come from separate historical uses or rebus borrowings, but the exact development is uncertain.
The flesh component ⺼ suggests something organic and sticky, like animal-based glue. 交 looks like a person crossing their legs, but here imagine it as a sticky substance being mixed and spread. Together, 胶 is a sticky glue or resin.
For コウ, picture a pot of glue that is so strong it could hold a whole cow together: cow -> コウ, and the glue binds everything.