I think although we never heard of Beethoven or Mozart, we can get a fair idea of what they sounded through the music composed based on their harmonies. I think this not restoration in proper sense rather this is more like creation of new music. Even though we cannot totally believe in the new versions but it can give us an idea. I think this is just like scientific creations of the dinosaurs and how they looked like. We do not know how dinosaurs might have looked but the DNA tracing and fossils of the dinosaurs can give us an idea of how dinosaurs looked like back in the ancient times