Wanted to make this post earlier, finally doing it now.
There are several tracks that are basically arrangements of traditional folk songs (there's plenty of info about this on the net of course). So anyway, a small list:
1. "Whispers of Oxenfurt" is an arrangement of the English song "Bonnie at Morn". CDPR posted an early version with the vocals here
http://en.cdprojektred.com/news/free...nt-soundtrack/
It's nice because I think this particular version isn't on the OST. There's "Bonnie at Morn (Instrumental)"
on the expanded score which is exactly the one, minus the vocals like it says. However, "Whispers of Oxenfurt" is a bit different version.
2. "Widow-maker" contains Croatian traditional "Naranča".
3. "Eyes of the Wolf" has part of Julian Tuwim's poem from the earlier Percival song "Oj dido", this is the only case with a known lyricist (Tuwim).
4. "...Steel for Humans" is the Bulgarian traditional song "Ой, Лазаре, Лазаре" (Oi, Lazare, Lazare).
5. "The Song of the Sword-Dancer" traditional Belarusian "Бегла старожа" (Begla starozha).
All the Slavic songs (excluding Bonnie) appeared on past Percival albums (for example Slava! Pieśni Słowian południowych, Reakcja pogańska) and were apparently re-arranged for the Witcher 3 OST. Some of Percival's non-OST renditions:
https://www.youtube.com/watch?v=5n6aLWOok2M
https://www.youtube.com/watch?v=iXnKxRvX3Z4
Anyway, it's possible I'm missing certain songs but this will do atm.