My limited RL knowledge of Welsh comes mainly from six months living in the English-speaking part of Wales, so I'm assuming I can't really rely even on what little Welsh I have heard (given that I lived in the Rhondda Valley and never heard the 'dd' of that name pronounced as 'th'!).
I've done what I can, based (very loosely) on the original and a bit of Googling of Welsh pronunciation guides, but if any passing Welsh speakers could have a look under the cut and let me know whether what I've come up with looks reasonable, I'd be enormously grateful. [I don't need it to be perfect, as I rather doubt the original was perfect, but I want to avoid anything in it being so wrong it's embarrassing!].
A is pronounced as in ‘man’
E is pronounced as in ‘bet’ or ‘echo’.
I is pronounced like the ‘ee’ in ‘queen’.
O is pronounced as in ‘lot’ or hot’.
U is pronounced like the ‘ea’ in ‘seat’.
W is pronounced ‘oo’, as in ‘zoo’. Thus ‘Mawr’ is pronounced ‘Maoor’
Y is pronounced ‘ee’ at the end of words and ‘uh’ (as in ‘under’) the rest of the time. Cymbrogi is thus pronounced ‘kuhmbrogee’.
Two adjoining vowels create a diphthong, which can change their pronunciation:
Ae, ai and au are pronounced ‘aye’ (as in ‘Aye, cap’n’).
Ei, eu and ey are pronounced ‘ay’ (as in ‘pray’).
Aw is pronounced ‘ow’ (as in ‘cow’).
Ew is pronounced ‘ayow’ (no direct equivalent in English, although the pronunciation of ‘you’ common in Birmingham, England, comes close).
Iw and uw are pronounced ‘ew’ (as in ‘yew’).
Ow is pronounced as in ‘tow’ or ‘low’.
Oe, oi and oy are pronounced ‘oy’, as in ‘toy’.
Wy is pronounced as in ‘wyvern’, or the ‘wi’ in ‘win’.
Ywy is pronounced as the ‘ui’ in ‘fluid’.
Consonants which differ from that which might be expected by speakers of English are:
C is always hard, as in ‘cat’ (not soft as in ‘city’).
Ch is aspirated, as in the Scottish word ‘loch’.
Dd is pronounced ‘th’, as at the beginning of the word ‘thought’ or the end of the name ‘Gwyneth’ (which is of Welsh origin).
F is pronounced ‘v’…or not at all if it is at the end of a word.
Ff, by contrast, is pronounced ‘f’.
G is hard, as in ‘goat’.
Ll is aspirated, with a burst of air like that made when pronouncing an ‘h’ in English. (Perhaps the nearest approximation for most English speakers is an ‘l’ with a ‘th’ in front.)
Ng is pronounced as in ‘finger’.
Rh is aspirated, rather as if the ‘h’ is pronounced before the ‘r’.
Si is pronounced ‘sh’.
Th is pronounced as in the word ‘thug’.
Ts is pronounced ‘sh’.