The speech machine is not reading this correctly, like more than 50% of the course.
The correct reading would be (ummun ðakiyyatun) or simply (ummun ðakiyyah).
The (-un) sound is called Tanwin or Nunation (you can check Wikipedia). For the time being you can think of this Tanwin as a marker for indefinite nouns, just like the a/an in English. Where "AL" (the definite article) is not present in a word, the Tanwin (Nunation) comes in at the end of the word. Tanwin can come in 3 flavors as well: -an, -in and -un.