If you think this looks familiar I did post in slack, but my tutor suggested this might be the right place for an answer.
I know that the pronunciation of w in welsh is quite often an oo sound (being a bit simplistic here, I know that isn’t quite right) as a vowel. However there are multiple instances where it is a hard w like in English - so for example
Cwrw
Mwy
Ddrwg
Are all the oo sound.
Ddwedest
Gwylio
Wener
Wedi
all hard w’s.
Is there a guide as to when its hard and when it’s soft?
Probably not a hard and fast rule, but I’d say if “w” is by itself it is an “oo” sound, and in combined vowels, if the w is short and unstressed, it is the english w sound.
(But in wy (egg), for example, the w holds the stress, so it is pronounced “OO-ee”)
Uh…are there really two different w sounds in Welsh and English?
Since I’ve never been aware of it, I’m not sure if trying to figure it out is going to help or make my language learning life more complicated at this point!
(Do you know if there are any specific audio files in case I decide to try?)
I agree with Hendrik, it’s not rally that the basic sound of ‘w’ is different, it’s just that if it’s pronounced quickly before another vowel which is unstressed the two combined make it sound as if it’s an English ‘w’'. … the stress is an added complication, though
The online Gweiadur dictionary has sound files for most words and it’s free but unfortunately there seems to be a limit on the numbers of people subascribing - might be worth tryng to see if they’ve increased the limit though
Looks like they haven’t.
They say to check Facebook and Twitter, but last updates are in 2016 and 2017 respectively. I doubt they’re still working on it.
Edit: in the meantime I’ve looked for more examples around the web.
I can actually hear the difference of sound for cwm, fwrdd, hwnt, cwpan - which seem closed and very short.
Yet different than those in @dee-4’s list and @Hendrik’s extra examples to me.
What about that?
Well, maybe the best advice might be not to be too worried about relatively small differences in the sounds. - when you’re speaking a lot of people may just think you come from a different area
There are bound to be differences between different parts of Wales (even to the extent, in parts of SW Wales, of the pronunciation of Oes as ‘Wes’ and coed as ‘cwed’ with an English ‘w’ sound ) - and there are very noticeable differences too in vowel sounds between different parts of England, although the pronounciation taught as the ‘standard’ pronunciationis from Southern England
It may interest people to know that, when I was studying ventriloquism, I learned that the way to pronounce ‘w’ in English as a vent is to say ‘oo’ - for example, ‘water’ becomes ‘oo-orter’ - which means your lips don’t move…
Funnily enough there are other parallels between Welsh and ventriloquism. For example, to say ‘violet’ you would substitute the English ‘v’ for the Welsh letter ‘dd’ - and to say ‘fudge’ you would substitute the ‘f’ for the Welsh letter ‘th’. I bet @siaronjames is going to try those now, too!
@siaronjames the key with vent is that you rarely pronounce single words on their own where there is a letter substitution as it can sound ‘contrived’. But in the middle or end of a sentence it sounds much more natural as the listener’s brain interprets the sentence as a whole and tends to ignore anything that sounds a bit less natural. That’s probably why the SSiW double-speed listening exercises work too (similar principle, I reckon). So if you say ‘thudge’ on its own, it might sound a bit odd, but try saying ‘I’d love a bit of thudge’ and you should notice it sounds almost completely natural. (Go on, try that one too!)
Rather boringly back to ‘oo’/‘w’ - my understanding was that the Welsh ‘w’ could have both the short and long Engilsh ‘oo’ sounds (think book and zoo), depending on the word, but when it comes before another vowel and is not part of a digraph, it then makes the English ‘w’ sound…
… the stress is an added complication, though 
- might be worth tryng to see if they’ve increased the limit though 
