I've found this example of the Breton pronunciation:
http://de.forvo.com/search/nolwenn/br/
And this one for the French pronunciation:
http://de.forvo.com/search/nolwenn/fr/
... and I can't vouch for neither of them ;)
Other than that I just got a French name book, and now I really feel like toying with
Nolwenn combos ;)