It's a sin to speak in a way that you lose self-control. With that said, there are certain times when it is a sin to be quiet also (see Bible verses below). Our reference point that governs us needs to be vindicating God's Truth and loving our neighbor acceptably according to Truth rather than our own preference and gratification.
James 1:26-27 "If any man among you seem to be religious, and bridleth not his tongue, but deceiveth his own heart, this man's religion is vain. Pure religion and undefiled before God and the Father is this, To visit the fatherless and widows in their affliction, and to keep himself unspotted from the world."
Proverbs 31:8-9 "Open thy mouth for the dumb in the cause of all such as are appointed to destruction. Open thy mouth, judge righteously, and plead the cause of the poor and needy." (yes, it is spoken to a king but we all have jurisdiction somewhere and ability to defend the poor and needy by speaking up somehow)
Mark 8:36-38 "For what shall it profit a man, if he shall gain the whole world, and lose his own soul? Or what shall a man give in exchange for his soul? Whosoever therefore shall be ashamed of me and of my words in this adulterous and sinful generation; of him also shall the Son of man be ashamed, when he cometh in the glory of his Father with the holy angels."