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?
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.