The song at the start of the Level 1 challenges is “Wela i efo llygad bach i” by Lleuwen Steffan - you can find the translation on this thread The intro song - 'Wela i efo llygad bach i'
The song at the start of the Level 2 and 3 challenges is “Coffi Du” by Gwibdaith Hen Frân, and there’s a video version with translation on this thread Level 3 South
As for what Iestyn says, do you mean the “Hwyl am y tro” ? (= goodbye for now)