We've all heard dinosaurs roar in all kinds of movies, TV shows and cartoons, but the question is if that has anything to do with reality?
And the short answer is most probably not.
But answering the question is actually quite hard, because almost everything we know about them is from their fossilized bones. This means a lot of the story remains hidden from us as all their soft tissues decayed before they could be mineralized. This means we don't have any voiceboxes left from dinosaurs to examine so we can determine what sounded like with certainty.
What we can do is guess based on what we know about their living family - their descendants and relatives that are living on the planet today. Those happen to be birds and crocodiles. Studying what sounds they can produce, what they have in common and what we know about their evolution can help us make an educated guess about the sounds dinosaurs were capable of.
Of course a lot of the sounds modern birds make have evolved long after they stopped being dinosaurs, so we can easily determine that chirping, quacking and clucking T-Rexes were highly unlikely. When we actually examine birds and crocodiles we'll find that their main approaches to making sounds differ quite significantly.
Birds produce their trademark sound using an organ called syrinx that is located at the bottom of their trachea while crocodiles have a larynx located in their throat like the larynx we have. Those distinctly different ways to produce sound have evolved after dinosaurs had long relinquished their domination over the planet, so it's safe to assume they had neither.
But there is a type of sound that both crocodiles and birds can produce which signifies aggression, but it's not what most people would expect from a dinosaur...
Hissing is actually a very simple way to make a sound in order to appear intimidating since it's basically letting out a lot of air all at once. Since this is something that both classes of dinosaur living relatives - birds and crocodiles - are capable of, we can say with a moderate level of certainty that dinosaurs could actually hiss too.
Additionally there is another form of sound production that is common to both birds and crocodiles which is called closed mouth vocalization. It employs producing sounds with a closed mouth or beak and using body cavities inside the trachea or esophagus to create resonance conditions. The resulting sound tends to be of low frequency and while some birds use it to coo or hoot, with creatures the size of dinosaurs, the result would be more like grumbling. It's important to say that this type of sound production evolved separately on multiple occasions so we can't really be sure if dinosaurs had also evolved that ability. Still, it is a fairly simple way to produce sound and it doesn't seem unlikely for it to have evolved for them too, so it remains a real possibility.
So to sum it all up, based on the limited information we are working with we can safely assume that dinosaurs most probably didn't roar, likely hissed, and possibly grumbled.
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