well almost, we don't know what they sounded like but we can make pretty decent educated guesses at what they probably sounded like in general.

For example parasaurolophus very definitely seems to have a resonating structure, like a trombone strapped to their face, so it'd be weird if they didn't make some sort of trumpeting sounds.

Another big one is that dinosaurs generally didn't have anything like a voicebox or whatever the thing is that birds use to make their calls, so we can be quite confident that most dinosaurs didn't make any bird-like noises, and they wouldn't have been able to do stuff like roar either.

Which leaves us with t.rex probably just having sounded somewhat like an alligator.