Author’s Note: Pronunciation of Celtic names and vocabulary in this series is based loosely on the contemporary Welsh:
VOWELS:
a is always a short A, as in can, ham, or man, never long, as in may. The Welsh words am and ac are pronounced as they would be in English.
e by itself is always as in get, pet, and let. However, the letter E has a different sound in the three diphthongs.
i has the I sound, as in bin or pin, or a long E sound, as in seen or queen.
o has the O sound, as in hot, or the long sound, as in toe.
u has the sound of long EE, as in see.
w has the sound of OO, as in boot and shoot, or of U, as in pull.
Note, however, that W can also be used as a consonant with the English W sound.
y has two different sounds. In one-syllable words (llyn), and in the last syllable of polysyllabic words (estyn), it is a shortened EE sound, as at the end of happy.
DIPHTHONGS:
Ae, Ai and Au are all pronounced as English eye.
Aw has the sound of ow as in how and now.
Eu and Ei are pronounced as long A, or the ay sound in say.
Ew is difficult for English speakers because there is no direct equivalent. It is approximately eh-oo or ow-oo, but the correct sound is between those examples.
Iw or I’w is ee-you with the ee sound very short. It is similar to the English yew.
Oe has the sound of oi or oy.
Ow is pronounced the same as in the English row, tow, or throw.
Wy has the sound of oo-ee or a short Wi sound, as in win.
Yw or Y’w is the same as Iw above.
Ywy (considered a diphthong, even though it has three letters) has the sound of ow-ee, as in the name Howie.
CONSONANTS:
B is the same as the English B, as in beer.
C is the Welsh K. It is always hard, as in can or cane, never soft, as in once.
Ch is a glottal Kh sound, as in the Scottish loch.
D is the same as the English D, as in dog.
Dd has the sound of voiced TH, as in this or there.
F always has the sound of V, as in have or very.
Ff is the same as the English F, as in first.
G is always hard, as in go or good, never soft, as in manage.
Ng has the English NG sound, as in singer, though in some words it has the NG+G sound of finger.
H is the same as the English H, but it is always pronounced, never silent.
L is the same as the English L, as in long.
Ll is a sound with no English equivalent. It is a voiceless alveolar lateral fricative formed by pronouncing L while allowing air to escape around the tongue; the English thl of athlete (or slat, pronounced with a lisp) is vaguely similar.
M is the same as the English M, as in many.
N is the same as the English N, as in no or never.
P is the same as the English P, as in poor or party.
Ph is the same as ff.
R is the same as the English R, as in right, but rolled.
Rh is pronounced as HR; that is, a slight H sound comes before the R sound.
S is the same as the English S, as in say.
Si is the same as the English Sh, as in show.
T is the same as the English T, as in turn.
Th is the English voiceless TH, as in think or three. Note difference from the voiced Dd.
W, when used as a consonant, has the English W sound, as in work.
Source: https://en.wiktionary.org