The thing is that they don't know. It's a puzzle with some educated guesses, based on some facts that they have managed to shake out of the dirt.

You see, dinosaurs and their skeletons have a lot of similarities to birds. Which means that you can make the educated - but difficult to prove or disprove - guess that a dinosaur would sound like a really huge bird.

Just by comparing a sparrow's sound to the sound of an ostrich, you get a hint of how size changes the sound. Apply similar reasoning to the size difference between an ostrich and a large dinosaur...and there IS a chance that they actually sounded kind of like it's pictured in a lot of movies.

Do we know for sure? No. Of course not. We cannot know for sure.

But guessing is fun too, when other scientists agrees that your guesses are not completely stupid.