Think about it

The stereotypical roars and growls are just sort of generic monster noises people thought a big “monster” would make. Later people came along and imagined up sounds based on all manner of things from large animals like elephants and whales, to simply deeper and louder sounds of common lizards or later birds. However some have conducted actual research based off fossil records to try and use acoustics and biology to recreate what certain dinosaurs may have sounded like based on certain assumptions and off of the existing anatomical information we have. It’s all still speculation in the end- but those are the more common bases used to create “dinosaur sounds.”

The T. rex roar from the original Jurassic Park film was a combination of a baby elephant’s squeal, an alligator’s gurgling, and a tiger’s snarl.