Q:

Why can Sona speak in game?

I enjoy reading the lore behind the champions in League of Legends. Sona is a Champion I enjoy playing so I read her lore earlier. It seems she is mute:

Sona has no memory of her true parents. As an infant, found abandoned on the doorstep of an Ionian adoption house, nestled atop an ancient instrument in an exquisite case of unknown origins. She was an unusually well-behaved child, always quiet and content. Her caretakers were sure she would find a home quickly, but it soon became apparent that what they mistook for uncommon geniality was actually an inability to speak or to produce any sound whatsoever. Sona remained at the adoption house until her teens, watching in hopeless silence as prospective adopters passed her by.

Yet in-game, she is able to speak. What's up with that?

A:

She is speaking directly to her summoner's mind. As she says when you select her:

"Only you can hear me Summoner, what masterpiece shall we play today?"

From the League of Legends wikia:

Because Sona cannot speak, these (her in game) responses are understood to be magical telepathy with her summoner. This is seen in the fact that her external interactions (taunt, joke) are musical cues rather than words, meaning that, as stated, only her summoner can hear her words.

Source: Link

