promptly, easily, i.e.
輒 centers on acting without delay or difficulty: doing something promptly, easily, or as a matter of course. The "i.e." sense extends this to clarifying something directly.
輒 combines 車 (vehicle) with 耴, which likely contributes the sound. The idea of a vehicle moving promptly may connect to the meaning, but the exact historical development is uncertain.
A vehicle (車) with a distinctive right side (耴) that looks like an ear (耳) and a hook (乙). Imagine a car that starts so promptly and easily that you just have to whisper in its ear and it goes.
For チョウ, picture a car that responds promptly to a command from a chief named Chou: Chou says "go" and the car moves instantly. Chou -> チョウ.