in a word, no, but scientists can make educated guesses. the closest living relatives of the dinosaurs are crocodilians and birds, and we can look to the ways they vocalize to give us a hint.
alligators and crocodiles use their larynxes to communicate—they’ll hiss, groan, and yes, roar (here’s a compilation of their sounds). dinosaurs might have had larynxes, but since those don’t fossilize, it’s impossible to know for sure. birds, meanwhile, use an organ called a syrinx, which seems to have evolved after dinosaurs. that might indicate that dinos couldn’t vocalize at all, which would be a bummer.
however, there’s also a possibility that they evolved a unique way to vocalize. for example, based on studying their skulls and inner ears, some have theorized that hadrosaurs used their crests to bellow at each other.
so, they probably didn’t roar, but bellowing can be pretty cool too, right?
the slowest thing is something that isn't alive.
i believe the molarity is 1.
molarity = number of moles / liters of solution
molarity = 3 / 3 = 1