If you're looking for manga similar to Tetsugaku Letra, you might like these titles. All recommendations are made by Anime-Planet users like you!
I wish we had never met. I wish we could meet once again. A boy who can hear, Shoya Ishida, and a transfer student who can’t, Shoko Nishimiya. One fateful day, the two meet, and Shoya leads the class in bullying Shoko. But before long, the class shifts its target from Shoko to Shoya. Years later, Shoya feels strongly that he must see Shoko once again...
Both manga center around similar themes and have a similar overall feel. The protagonists have the same sort of akwardness, and both mean well even when what they do isn't necessarily right. Both manga feature bullying, thouh in different ways and to different extents. Both manga deal with moving on from your past and trying to overcome emotions. The mcs also both try to give up at various points in the stories, but it turns out differently from how they expected. They also both have a romance built into the manga, but the romance isn't what drives the plot and doesn't really take center stage in the overall story. Both also have a lot of character developement, and are very character driven in general.
A Silent Voice and Tetsugaku Letra both have very similar characters and a similar feel overall. Despite having quite different themes, they are fundamentally very similar, and if you liked one you'll probably like the other.