The Sound of a Dinosaur?

How does a dinosaur sound, does anyone have the answer to that question. How do the scientists know, or are they just guessing? I mean can you really tell from a skeleton...So theoretically they could sound like a bird or a pig, right? Does one of the closest relatives, the crocodile make a sound that resembles to something outta Juric Park, if so I've never heard one that does.