We have written pieces of music from ancient cultures, but we don't know exactly how these forms of music notation worked. There's no Rosetta stone-type document that shows how, for instance, Greek or Roman music translates into modern notation. Likewise we don't know what scales they used, tunings of pitches, how instruments were pitched, and so on.

That being said, a lot of music historians have attempted to extrapolate what the notation says, what the likely tuning of intruments would have been, built reproductions as closely as possible to depictions in art, and to artifacts discovered, figure out the technique as to how they were played, etc., so the reproductions are as good as they can be, but is still educated guesswork, so yeah, that's all we have.