How do I get mod podge out of a sponge brush that’s already hardened?

Since Mod Podge is water soluble, you just soak the brush's bristles, keeping the ferrule (the metal band) and the handle out of the water. In a half hour it should be soft enough to scrape off with your fingernails.

You can't.

Since sponge brushes are super cheap just toss it and buy a package of new ones.

soak it in gas.