I noticed that the T-Rex roar wasn't only missing in the boss fights against him in Dino Crisis 2 (which was fixed a few days after release), but it's also missing in Extra Crisis. Both in Dino Colosseum and in Dino Duel you can press L2 (on a PlayStation controller) when you play as a dinosaur, and it will roar. When you do it with the T-Rex you can only hear the growl, but I watched a video of the PS1 version, and I noticed that there is also supposed to be a roar when you press the button.

Here is a video showing that the T-Rex is supposed to roar when you press L2 in Extra Crisis. At 0:57 and 1:23 for example.
https://www.youtube.com/watch?v=pAwa6m_n8C8