The -un sound at the end of Rawwaad(-un) and sa3iid(-un) is a nominative marker. If a word begins with the definite article, it loses the final -n and it becomes -u. Native speakers frequently omit those endings, and they're almost always omitted at the end of the utterance, hence why kariim(-un) isn't pronounced with the ending in this sentence.
Probably because they didn't intend for the case endings to be taught in the course to begin with, probably thought it was too hard for beginners, but then the TTS engine decided to pronounce them on its own. You really shouldn't trust the endings that you hear in this course, because soooo many of them are wrong, since the contributors didn't bother to actually mark them in writing and the computer has to guess.