Jurassic Park Questions.
 

This was a question I've been wondering for a long time. When they made Jurassic Park, how did they know what T-Rex sounded like? Also in Jurassic Park 3, how did they figure out that the Spinosaurus had a sail on its back?

Sounds are just best guesses. They can estimate based on nasal passages and modern equivalents. Spinosaurs' sails are the most popular theory for the purpose of the bone structure that appears on their backs.

As for the sound of the t-rex, and all the other dinosaurs too, it is a complete guess. Basically they just created a sound that they thought sounded good for the movie. For the t-rex, they actually combined the sounds of a bull elephant, a grizzly bear, and a lion.

The spinosaurus' sail though, is based on fossil evidence. No complete spinosaurus skeletons have been found, but as was already mentioned, the sail is the best theory as to my the vertebrae were so tall. Check out the wikipedia page for spinosaurus for more info on that.

Spinosaurus
 

The remains of spinosaurus were discovered by ernst stromer in the 1900's The sail itself was part of the first remains. Of course the sail and other parts we're destroyed when the natural history museum in munich was bombed In the second world war. The photographs of the spine are what is used at the moment. but another species of spinosaurus was supposedly found in morroco (other than S.Agypticus) so there is a possibility that another sail could be found, an articulated skeleton would be amazing though