I don’t really like her because of her harsh tone.
Koichi said, in a very stern voice, that luck is just part of his talents.
She scolded me violently in a severe tone.
When your mouth makes tones we call it your tone or tone of voice. They're different kinds of sounds you make with your mouth that make you sound happy, sad, mean, etc.
This is a jukugo word that uses the on'yomi readings of the kanji. You haven't learned this reading for 口 yet. Remember when you learned the vocabulary word 口? Just take the ち off and you get its other on'yomi reading く. That's easy, right?