Thanks to
howlingquietly, here's a tiny sound clip from Delta and the Bannermen 2, including Burton and Ray's exchange in Welsh. I'm now pretty sure she says, "O, mae'n wir, mae'n wir" - "Oh, it's true, it's true!" - but I still have no idea what he says!
http://www.megaupload.com/?d=FKJXG7O5 ETA: omg! Burton says: "Dangoswch imi!" - "Show me!" (Probably a better rendering of Ray's line, then, is "It's the truth!")