They're dominant in Douai, but rarer in Rhyl (6)
I believe the answer is:
vowels
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for vowels that I've seen before include "A e i o u" , "Speech sounds made with the vocal tract open" , "aeiou" , "Five speech sounds" , "'Five letters, not consonants (6)'" .)