If you ask most current respectable paleontologists who specialize in dinosaurs, they would most likely hiss and chirp, similar to lizards, crocodilians, and snakes. 

That’s just the most similar extant species morphologically. Some of the bird like dinosaurs would probably sound like birds to a certain extent.

Of course, hisses and chirps would sounds very different if you scaled them up and made them more bass-y in proportion to a larger sauropod, T-Rex, etc.

They could also use different sounds based on other features, like clacking jaws/beaks in rhythms, or some other bird like behaviors.