This sounds like Akata Witch, by Nnedi Okorafor (depending on your location, it may also have been titled "What Sunny Saw In the Flames").
"Leopard People" (that is, people with innate magical talent) often use juju knives to work their magic spells, and, as you remember, the knives often choose the user.
“Who cares?” he said. “You want a juju knife, right?”
She nodded, grinning. She liked Junk Man very much.
“Close your eyes, reach in there, and pick one up.”
She shut her eyes. As she rummaged around, one of the knives cut her. “Ah!” She snatched her hand away and openedher eyes.
Junk Man immediately reached into the box. “We have a winner,” he said. The knife he brought out had a small smear of her blood on the blade. “Funny,” he said.
She stared at it. “What is that?”
“Oh. Weird,” Orlu said.
“Is that the one that chose you?” Chichi asked, coming over.
“Oh, that’s—uh, that’s different,” Sasha said.
Its handle was an unremarkable smooth silver, but the blade was paper-thin, made of a clear green material, like glass.
“Man from the north gave me this one for free after I bought some others from him,” Junk Man said. “He wore a thick veil, so I didn’t see his face. But he had eyes pretty like a woman’s and a very kind voice. You can always tell a man’s nature by his voice, a woman’s nature is more in the eyes. Anyway, there’s your knife. It picked you fair and square.
The quote you partially remember comes from the main character witnessing the commentator on a wrestling competition at a Leopard People celebration, using her knife as a spell to amplify her voice:
A regal woman briskly walked onto the field. She brought out her juju knife and Sunny nearly screamed with horror as she dragged it across her throat. Then she remembered where she was. There was no blood, not even a cut.
“My name is Mballa and I will be your commentator this fine day,” the woman said in a highly amplified voice. “Welcome to the two hundred and forty-sixth annual Zuma International Wrestling Finals.