So does the text not match the audio?
Or is it normal to pronounce it differently than if the word would stand alone?
We often don't pronounce diacritics at the end of sentences, and the makers of the course are trying to strike a balance between Modern Standard Arabic and spoken Arabic, so they sometimes drop that sound, too.
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.
So, @Umm_Yahya's version with diacritic in the end would be "baab un kabiir un"?