It is supposed to be pronounced like the Greek upsilon ([face=SPIonic]U[/face]) which it represents. That letter is to be pronounced like a French 'u'. There is unfortunately no way to express this in English terms, but you can probably get someone to make the sound for you or find a recording. As with all Latin nouns, the macronized version is about twice as long as the short version.
flebile nescio quid queritur lyra, flebile lingua murmurat exanimis, respondent flebile ripae