It's confusing, though. With this sentence, the "nunation" is used in the sentence in the exercise but not above in this discussion's "blue" sentence; and the tanwiin (vowel) was not even shown when the "-un" sound was made.
When I clicked on the blue sentence at the top of this page, three phrases were pronounced in Arabic: big garage, big jacket, and big house. Garage and house had nunation but jacket did not, and there's no explanation why two had it and one did not. Is there a grammatical reason, is it a mistake, are they trying to show us both ways? But when the sentences are all "shuffled-up", why is an explanation not given to us about the reason for the shuffling?
We're playing with a deck of different grammatical rules and different pronunciations all shuffled together and no instructions on how the game is to be played.
The -un signals nominative case for indefinite singular nouns (and adjectives), effectively corresponding to the English a/an. For definite nouns, the ending is -u. So: al kitaab-u: the book, kitaab-un: a book. In colloquial Arabic (dialects, as opposed to standard varieties), the endings are left out (with some exceptions), so: al kitaab: the book, kitaab: a book. In more general terms, this really begs the question of which variety of Arabic should be taught: standard (not really spoken anywhere but recognised everywhere) or dialect (targeting the spoken language of a specific community).
Right now, I too have absolutely no sound anywhere on this lesson. But, since Duolingo taught me so well, I've been able to read the sentences and select or write the correct answers. But, this is not how it should be. Please fix the sound quickly Duolingo because some questions can be "write what you hear" and reading won't help us there.