That's always the problem with music from the medieval and early renaissance. Sometimes there's no music, or if there is, it's very sketchy. Take the case of the castle decoration.
Really. In the process of studying some 16th century carvings on Stirling Castle, Scotland for restoration, scholars discovered an unusual pattern circling the medallions. They turned out to be musical notations. The three symbols indicated different notes. And (as you can hear on the BBC website), they work.
And that's how modern musicians recreate the music of the distant past. As Huw Williams of the BBC wrote,
So what we're actually hearing is a combination of sound musical scholarship and educated guess-work.The music might not be quite the same as it was performed in the 1500's, but it still sounds pretty good to me.