the reason why rengoku (demon slayer) was laughing talking loudly

1.  rengoku first mission:-

this was the reason why he  talk and laughed loudly because he had  hearing problems.

   During his first mission as a demon slayer rengoku faced off against the flute demon

but the demon turned out to be too strong than a normal demon. he can control the opponent's 

body with the sound of his flute, many of rengoku comrades fell among them was rengoku's friend who

sacrificed himself. rengoku didn't know what was going on but his comrades who could control their

bodies a little bit used sign language about demons' abilities. he hit his ear so hard that it damaged

him so he could no longer hear anything after that rengoku easily defeated the demon because his 

ears were no longer under the manipulation of the demon's flute. rengoku accomplished his mission

 but his ear never fully recovered.

    

let's discuss about flute demon 

RACE:- Demon; human(formerly)

NAME:-FLUTE Demon, fue oni (unofficially)

 lets  discuss about flute demon

Its name is either unknown, unofficially translated, or based on conjecture.The Flute Demon ( 籥 ふえ 鬼 おに , Fue Oni?)

 was a Demon exclusive to the Special One-Shot who desired to join the Twelve Kizuki?

 He was responsible for the deaths of a small group of Demon Slayers and innocent civilians.

Appearance

The Flute Demon takes the appearance of a very elderly man with wrinkly skin, and incredibly long black downward-slanted eyebrows. He has long slanted eyes with black scleras similar to the Serpent Demon with the traditional fangs of a demon.

His attire consisted of a flower-patterned kataginu-sleeveless robe tied to one side by a white ribbon and long white kimono with a thick black frame at the cuffs. He has long black  pants similar in appearance to the Demon Slayer Corps members with geta shoes. As well as this, he wears a black Japanese saggy cap with a tassel at the end.

Abilities

Overall Abilities: Due to him being able to easily evade Kyojuro's  first attack as well as him killing nine Demon Slayers, he was incredibly powerful compared to the average demon. In addition, due to him stating that he would soon join the Twelve Kizuki, it can be assumed that his strength is near equal to that of a Lower Rank.