"قَهْوَة سيث طَيِّبة."
Translation:Seth's coffee is good.
In Standard Arabic Grammar, mostly indefinite nouns (nakira نكرة) have -un, -an, and -in ending sounds while a lot of definite nouns (ma3rifa معرفة) have -u, -a, and -i endings. That is actually the most correct way of Arabic pronunciation. You may check this matter in Quran where all letters get the diacritic marks properly. (It is recommended for beginner students to study the endings for further understanding).
However, in Duolingo, Arabic contributors try to omit all endings as they are not spelled in the direct conversation. As far as I've met the endings here, some are correct and some are incorrect -- but, mostly they are incorrect (Perhaps, they are audio glitches. We just try to build our positive thinking because: if these errors are not audio glitches, then contributors don't understand the endings).