The "unbroken tradition" is often mentioned, and it is a very fine thing. But you need to be wary of it, and not give it undue weight. For example, I may sing the odd song I learnt from my grandfather, and maybe he got some of them from his grandfather. But that does not mean that the way I play and sing has any particular similarity to the way my great-great grandfather did. It might do, or it mught not. So you would be exceedingly foolish to assume anything from it about how people sung a generation or two back.
That is why early 20th century recordings are so valuable and interesting if you like folk music. So much has been overlayed onto Welsh music by the Eisteddfod on the one hand, and the folk revival(and the associated "pan-Celtic" industry) on the other, that it is very very difficult to get a handle on what Welsh raditional music actually sounded like.
    On a totally different note(and a bit of thread creep to another Welsh tradition), my grandfather, Dai Gent, invented the concept of the specialits scrum and fly halves(as opposed to the left and right halves, which had been the practise hitherto). There, I knew you'ld find that interesting.