Honestly, I didn't really understand the title when I first saw it, and it's still not completely clear to me now, but I'm not a native speaker so I can't judge for native speakers. Still, I like how the title sounds, and the 'silent' has definitely a deeper meaning when you think of Jungkook's illness (I guess it's an illness?), and it's nothing common or boring so I actually really like it. It also made me curious to read the story because I didn't understand it and wanted to know what it means.

The foreword describes what the story is about well, though it makes even more sense when you already know the first three chapters, but it sounds interesting and I also like how you introduced the characters before the first chapter because it's easier to follow the storyline then.

I think the grey colour and the pianist in the poster fit the storyline and Jungkook's character perfectly fine. Jungkook smiling (he's the person on the left, I guess because I don't know their faces that well) however doesn't suit Jungkook in the first three chapters so far, and though I suspect Jungkook will be much happier later (just a feeling), it's your decision if you want the poster to fit Jungkook in the beginning or later, and just in case you want to portray Jungkook in the beginning a sad smile might fit better - but this is your decision, I just don't know the end of the story and so far I imagine Jungkook as a sad pianist. Of course, if this poster is designed for later, this looks like Jungkook is sincerely smiling and would portray his progress good - depends on what you intended.
Taehyung looks like he's trying to cheer someone up, and that fits him volunteering at the hospital as well as maybe cheering Jungkook up (I just guess he will do that later) so if this poster is designed for later it would be nice to see Taehyung trying his best to cheer up Jungkook, and Jungkook sincerely smiling because of him, but I just know the first three chapters so I officially don't know how the story will go on.
The graphics of each chapter (the white card with the colours) are looking nice, though they don't have anything to do with the story so I don't really think this is something bad, but maybe some readers might prefer story related graphics like a piano motive or something, but honestly I am okay with the colours on a white background.

I can't judge the character development that well because the story is not completed yet and a lot will happen in the future, but the characters are really interesting, especially Jungkook because he's portrayed very unique and realistic. I also like Hoseok as an older brother who worries about Jungkook (because Jungkook really needs an older brother who helps him to socialize a bit here) and Taehyung as a cheerful photographer who will make Jungkook realize there's more than music in his life, the characters are all really realistic and interesting and fit the story well. [I just judged the characters here because I can't say anything about the development so far]

The font looks completely simple and normal and is easy to read, neither to small nor too large, I didn't have any problems with reading. The structure is perfectly clear and organized so it's easy to follow the storyline.

This story is definitely really creative, and I haven't read something about the main character being a pianist who loses his ability to hear yet, and this is definitely not a common plot, and both meeting through volunteer work is new to me too.

The flow is really good, it's neither too fast nor too slow and the things happen in a realistic time (many authors have problems with that but your story is perfect in that aspect). The events happen in a nice structured order and the storyline is really 'flowing'.

Your writing is definitely good, it's perfectly understandable and I really like the way you describe everything.

The spelling is perfectly fine, I don't really remember finding typos (there is 'voicse' in the beginning of chapter two), and your grammar is really good too, especially because you're not a native speaker, sometimes you wrote in present tense though it should have been past, but in general, it was really good.

I really enjoyed reading these three chapters and also want to know how it will continue because I really like your characters and the storyline, and the way how you describe situations is really amazing ^.^ I really like the scene where Taehyung takes Jungkook to this place in the forest and Jungkook decides he can trust Taehyung enough to tell him because it's definitely a development for usually-not-so-social Jungkook, and the checklist is interesting too, though I didn't expect learning photography to be on the list.