The title says it all, how do you say "speak one's piece" in Spanish?
 Speak your piece means "state your opinion or view".
There is a debate if the correct phrase is "Speak one's piece" or "Speak one's peace". I have no intentions to go into details here, so I'm going to assume they are equivalent, or otherwise as explained here.
The example the book gives is:
It references someone's need to say something. I agree with the book, it would just translate to "decir lo que se tiene que decir" or "decir lo que se quiere decir".
It's very difficult to make a translation without any context or even a full sentence.
Let's imagine the sentence in English was:
then a translation into Spanish could be: