Question

Which line from Extremely Loud and Incredibly Close reveals a melancholy tone?
As for the bracelet Mom wore to the funeral, what I did was I converted Dad’s last voice message into Morse code, and I used sky-blue beads for silence, maroon beads for breaks between letters . . .
She said it was the best gift she’d ever received. I asked her if it was better than the Edible Tsunami, from when I was interested in edible meteorological events. She said, “Different.”
I wanted to tell her she shouldn’t be playing Scrabble yet. Or looking in the mirror. Or turning the stereo any louder than what you needed just to hear it. It wasn’t fair to Dad, and it wasn’t fair to me.
I made her other Morse code jewelry with Dad’s messages—a necklace, an anklet, some dangly earrings, a tiara—but the bracelet was definitely the most beautiful, probably because it was the last, which made it the most precious.

Answer 1

"She said it was the best gift she’d ever received."

The sentence reveals the Mom's sadness over the death of your Dad. But she also felt his presence and voice through the bracelet which made her strong enough to deal with the situation.

Answer 2

The answer is:

I wanted to tell her she shouldn’t be playing Scrabble yet. Or looking in the mirror. Or turning the stereo any louder than what you needed just to hear it. It wasn’t fair to Dad, and it wasn’t fair to me.

The tone of a literary work is the attitude of the author towards certain topic. Therefore, a melancholy tone refers to a feeling of sadness and serious thought.

The lines from "Extremely Loud and Incredibly Close," by Jonathan Safran Foer, express Oskar's difficult relationship with his mother. In that respect, he still mourns his father and wishes she would not try to enjoy herself.