It is true that he only talked once in the whole intire TFP series but the question still remains, why is Soundwave mute?, what happened in his past?, and who hurt him to make him go mute and stop talking?
Yes, he only spoke four words in the whole series. I’m pretty sure his silence has something to do with trying to prove a point. Like actions speak louder than words or something. Don’t hold me on it though.
Soundwave is not mute. He decided not to speak. He follows idea that action speak louder than words (maybe his experience from the time when he was member of the council). In TFP he really talked only once and he said one sentence which soundwave's fan remember (soundwave superor, autobots inferiors).
I feel sad for you soundwave I don't know if I can help you as your best fan even to starscream and the autobots, but I will help you get out of your mad world and help you feel better!