We may think we know what dinosaurs like the T-Rex sounded like; scary, imposing, and full of deep-throated roars. However, researchers at Midwestern University are questioning that long-held belief, often made popular by pop culture, with a few studies of their own.
Lead researcher Tobias Riede and his team took Hollywood to task by researching the evolutionary history of birds, which are said to be the closest modern day relatives to dinosaurs. Birds often vocalize with an open beak that can create a wide range of frequencies, but none as daunting as the movie version of a T-Rex.
Many also often vocalize with their beak closed. This creates much lower frequency birdcalls, from hoots to booms. The lowest frequency birdcall belongs to the cassowary, a large bird that cannot fly. This boom it emits may have been similar to that of some types of dinosaurs.
The researchers studied the sounds of more than 200 archosaurs, which include animals such as birds and crocodiles, in order to try and come up with a conclusion that might make sense in terms of dinosaur sounds.
While educated guesses can be made on the noises they emitted, scientists recognize that they will never know for sure. Similarly, they will never know for sure if dinosaurs were covered in scales as seen in most popular films, or if they were covered in feathers like their current avian descendants. This is because the soft tissue of the dinosaur’s vocal chords (if they had them at all, which many scientists doubt), or where in their body they would emit a noise from, does not fossilize in the same way bones do.
Scientists, however, theorized that many non-avian dinosaurs may have actually resembled the calls of ostriches and cassowaries instead of their Hollywood representations. 
 Washington Post