Quick question from a young friend of mine who’s interested in studying linguistics at university, and currently on her way to a week’s work experience in Bangor: “are there different pronunciations, across Wales, for the ‘w’ in ‘dwy’ and ‘blwyddyn’?”
My instinct is that I use approximately the same vowel in both, but that the -wy- of ‘blwyddyn’ is shorter (southern English learner, did Gog courses here). Any other thoughts from those more knowledgable?
Saying them out loud with my North Wales accent, I notice slightly that the dd in blwyddyn softens the w sound slightly and that I tend to emphasise the w more in dwy.
Also, I would note a difference in the vowel sounds of these two words in north and south Welsh, but pertaining more to the y rather than the w.
In North Wales, the y would be more like a u/uh sound, especially in the Pen Llyn and Caernarfon areas, but in South Wales, the sound is more like an ee.
Hope this makes sense! 
Diolch am hyn! I also asked elsewhere (Gramadeg y Gymraeg group on Facebook) and got much the same answer, but with added IPA and whatever the Welsh for a near-front near-close unrounded vowel is. Or, indeed, the English.
But the interesting thing to me was that there is, apparently, a length difference - in the North, but not the South - in that -wy- sounds longer in some single-syllable words (depending on the consonant, if any, on the end) - and that that is the pronunciation I’d unconsciously picked up from SSiW. I had to think about how I say those words, to try to answer the question - and it turns out that I was making a correct and specifically Gog distinction without even realising it 
(I’m thinking swydd long, vs swyddfa shorter; rhwyf vs rhwyfo etc.)