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.
As a starter, in Arabic, you cannot start a nominal sentence (sentences starting with a noun) with an indefinite noun. Thus, sentences like a mother is smart cannot be formulated as it is in Arabic. Now, the translation for such sentences would depend on the context in which it occurs. Most like, it would be forced to be: the mother is smart.
Now, we have here two cases in English: An attributive adjective, and a predicative adjective. The Attributive adjective is the adjective that describes the noun directly by attaching itself to the noun; As is the case of smart mother. The adjective "smart" is attached to the noun "mother".
The predicative adjective, on the other hand, is an adjective that "tells" the status of the noun and typically in English we would need the verb "to be" to complete such information or sentence, as in the mother IS smart - "is" here is the auxiliary verb that connects the subject to the predicative.
Now, the same concept of attributive and predicative is applied in Arabic. However, the method is different.
First of all, adjectives in Arabic always come AFTER the noun (I believe just like the Romance and Celtic languages). The attributive adjective in Arabic comes after the noun and FOLLOWS is attributes: gender, number, and definition. Thus THE smart mother is الأم الذكية (al-ummu al-Thakiyyah), while A smart mother is أمٌّ ذكية (ummun Thakiyyah) - The Tanwin (-un) is like an indefinite marker for nouns as is the case with the English (a/an). Tanwin also has other grammatical uses but that's another story.
Now, the predicative adjective, follows the noun in number and gender, BUT NOT in definition. The predicative adjective is ALWAYS indefinite. As I've stated before, sentences like a mother is smart are not possible in Arabic, and this would be forced to be defined and changed to the mother is smart - This sentence would be الأم ذكية (al-ummu Thakiyyah). We see clearly here how the adjective ذكية is not defined while the noun before it, الأم is defined, clearly shows that this adjective is not attributive and is not attached to the noun, but it is more likely to be a predicative adjective telling information about the status of the mother.
So, in a nutshell, it is the definite article (AL) that plays the role here of changing the class of the two words, from a phrase (noun+a.adjective) to a full sentence (noun+p.adjective). Arabic, like Russian, does not use the verb "to be" in the present tense to connect elements of the sentence and it is possible to have a full comprehensible sentence without using a verb in the sentence, unlike many languages of Europe.