My friend I will not lie I didn't even noticed such detail
Well played to you
Well, the title is called doorbell so i imagine the irl doors would be important too not just aAzsura's 'key' as well as the other comments there, but then again considering the classmate went home and immediately got into the situation/saw his mom like that, he wouldn't have focused on locking the door, if anything you'd expect it to be left wide open for the neighbors to overhear/report it
(tho the 'click' sfx could just be the doorknob turning, since the chain blocker wasn't used either but idk if Asura would have the strength to knock down a door even if he's gotten more confident since the beginning, i do still half expect someone in the future to ambush him and steal his key, assuming it wouldn't 'pop' back in his pocket magically or only being able to be used by him)