> a mass shift towards singing with tremor by peasants or
> weavers

One possibility I've heard for this impression is that said old folk were simply not very good singers (my €0.02: their voices may well have also deteriorated with age and/or lack of practice). Then revivalists, listening to the field recordings, picked up the quavery pitch from them, the same way English youngsters tend to pick up Yankee accents (and that annoying nasal whine) from pop records.

But this sub-discussion belongs in a different thread. We now return you to our Subject Line.