Montag lives in a society where not only speech is controlled (through the burning of books, and the production of only informational materials), but also thoughts. Controlling speech helps to control thoughts, and as the kids in schools are crammed full of factual, useless information, then kept busy watching t.v. and smashing cars on the weekend, they have no room or time for true thinking. Clariss mentions that at school
"we never ask questions; they just run the answers at you, bing, bing, bing."
This indicates that speech--and thought--is being controlled by the teachers. They give all of the answers and information the kids need so that kids can't or don't need to ask questions at all, or express curiosity through speech. And, since Clarisse is a curious girl, and very happy being so, Bradbury seems to be indicating that the repression of speech and thought leads to shallow, violent, non-connected human beings who just follow whatever orders are given to them.
Beatty's speech to Montag gives more background on how speech and expression of thought through words is repressed. Beatty states society's attitude towards people who expressed themselves through writing and thought:
"Authors, full of evil thoughts lock up your typewriters...Books, so the...critics said, were dishwater...a book is a loaded gun next door. Burn it...breach man's mind. Who knows who might be the target of the well-read man? Me? I won't stomach them for a minute."
Through Montag's society, Bradbury is pointing out what happens in our society all too often. Expression of speech allows mankind to express individuality, and views that might contradict the powers to be, or cause rebellion and dissatisfaction in the masses. In order to control the masses, we control the information. We don't report on protest rallies that occur. We don't allow certain books or news items to be published. We discourage and defame anyone who speaks anything against what the powers that be want to be heard. Bradbury, through the use of this dystopia, indicates that the lack of free speech in society means the lack of discussion, progress, thinking minds, individuality and freedom.
I hope that these thoughts help; good luck!