If you look them up, I am sure there is supposed to be a difference, but is the audio system accurate enough to distinguish them in these exercises? I'm not convinced.
It would be awesome to have some minimal pair exercises (i.e. presenting la rue and la roue one right after the other) for training just this sort of thing.
Yes, I can clearly hear the difference on Duolingo. You really have to train your ear to hear it, though. It took me a few months of being immersed in French in Québec before the difference became apparent to me.
I suggest you compare the pronunciation of the following words on Forvo.com and/or Google Translate:
roue/rue, dessous/dessus, vous/vu, bout/but, nous/nu, etc.
Agreed that there should be lessons on this to work on hearing the difference.
IMHO Acapela.com is an easier site for pronunciation I think... unless I wasn't navigating the Forvo site correctly (first time there). Acapela will say each word one after the other by typing them in. With Forvo, you type one in listen, and then type the other in and listen. the small time gap makes it harder to distinguish the difference.
Yes, there is a difference. The U sound in "rue" doesn't exist in English (or we don't distinguish between it and OO/OU). It's a "tighter" sound that's made by moving the bottom lip up a bit from the "OO" sound (as in "food"), and pushing air forward slightly more forcefully than you would with "OO." It's difficult to explain, but try typing "rue" and "roue" into Google Translate and listen to the difference. Try also typing "pur" and "pour" or "bouche" and "bûche." Note that this is the sound made whenever you have a U either on its own, or as the first vowel in a diphthong (I believe that's what it is, anyway), so the tight U sound is used in words like "tu" and "du" which are not homophones with the English words "too" and "due".