(It's canon to the Demon Slayer series.Zenitsu's hearing is the best out of the other demon slayers.His hearing is equal to Tengen Uzui's hearing.)

Anyway,let's get to the important things now.

Also,it's raining outside.

Tanjiro and Inosuke needed to go on a mission and left Zenitsu at where the demon slayer corps lived.Zenitsu stayed at the entrance of the butterfly mansion,sittied down at the entrance and observing the rain. (You're Zenitsu.)