how did scientists know what sound dinosaurs make and what their skin looked like?

I did hear a report that a well preserved large dinosaur showed feathers and a quite reddish skin.

Feathers on dinosaurs is well established now, up at the front end.

It is not known if T-Rex actually had feathers. However, T-Rex was a theropod dinosaur, which comprises a very large group of dinosaurs, from quite large to small types. The fossil evidence indicates that birds evolved from  a small, feathered theropod dinosaur.

It is now clear from their bone structure from the fossil evidence that at least some branches of the dinosaurs were warm blooded and much more active than their reptile ancestors. Numerous examples from the fossil record also show that many of these dinosaurs had evolved feathers, which would have originally been used for insulation. From that initial use other uses would have arisen, such as for camouflage or elongated for sexual display. Some of the smaller tree-climbing feathered theropod dinosaurs began using their elongated feathers on their arms for gliding. Eventually, they evolved into powered flyers.

The earliest example of such feathered, winged theropod dinosaurs was that of Archaeopteryx, several fossils of which were found in the 1800s. It is clear that the skeleton is still more like that of a theropod dinosaur than like that of a modern bird, including having teeth, a long bony tail, and separate forelimb digits with claws instead of the fused digits of modern-type birds.

Fossils of numerous other small, feathered, dinosaurs, including winged, have been found in the past thirty years, enough to make it clear that birds descended from feathered theropod dinosaurs.

Do a search on YouTube for "Microraptor: The Flying Dinosaur | Planet Dinosaur | BBC Earth" and you will see examples of a couple of other feathered winged dinosaurs. (Y!A is currently having a problem when links are included within an answer.)

Also, do a Google search for feathered dinosaurs and you will find more examples.

These early ancestors of birds first arose over 150 million years ago and gradually evolved into modern-type birds before the end of the Cretaceous (which marks the demise of the non-avian dinosaurs). NO fossils of any modern-type bird is found in earlier Cretaceous strata. Furthermore, NO fossils of present-day species of birds appear in strata laid down in even the late Cretaceous, though a few examples of some possible ancestors of ducks appear in late Cretaceous strata. However they might just be examples of convergent evolution as an adaptation to similar environments.

For that matter, absolutely NO fossils of present-day species of mammals, including human, appear in the same strata with dinosaur fossils anywhere in the world. That is because the present-day species of mammals did not evolve until LONG after the dinosaurs died out as a result of an asteroid impact (for which there is a great deal of evidence) on the earth 66 million years ago. The only mammal fossils to be found in the same strata with dinosaur fossils are of relatively small, early types having no clear relationship to present-day species.

Is There Evidence of Feathered Dinosaurs? https://creation.com/media-center/youtube/ct-evide...

We have more than bones to go on. Sometimes we have skin imprints. Archaeopteryx (yes, I know, a bird) showed clear imprints of feathers and some of the small Chinese fossils show imprints of elongate scales; there are a few paleontologists who think that those scales are feathers.

Sounds are easy to guess at from the bony structure. A good musician (brass or woodwind) should be able to come up with a rough estimate based on the size and shape of the resonating parts.

Some is speculation based on existing descendants.  Some is based on fossilized imprints found in mud beds (ancient ones). And some is just a guess.