James 3:4 reads "Behold also the ships, which though they be so great, and are driven of fierce winds, yet are they turned about with a very small helm, whithersoever the governor listeth." Just as a boat can be turned by a small part so can our words significantly effect our lives and the lives of others.
In the same chapter of James in verse 5 it says, "Even so the tongue is a little member, and boasteth great things. Behold, how great a matter a little fire kindleth!" Fire is another great analogy for our mouths. Just a small flame can start the biggest forest fire. I'm sure we've all seen this happen before. Just look at what gossip can do...
Another part of speaking doesn't necessarily have to do with what is said, but how it is said. I would encourage you to read a talk by President Thomas S. Monson entitled, "School Thy Feelings, O My Brother." It's subtitle reads: If we desire to have a proper spirit with us at all times, we must choose to refrain from becoming angry.
I am grateful for the words of and voices of many people. I'm especially thankful for the words of prophets that show us the way.