Indo2Know contains the following sections
The alphabet section just plays the spoken letter when tapped.
The Word List section provides a full list of over 1,200 Indonesian words, with their English translation. Selecting a word will play a recording of the spoken word. It is also possible to search within each list, by the Indonesian or English word.
The Audio Card Quiz section consists of multiple choice quizzes where you will hear the spoken word.
The Flash Card Quiz section consists of multiple choice quizzes where you can choose between getting the question in English or Indonesian. When you select Indonesian questions, it is also possible to hear the word spoken.
The Writing Quiz section consists of getting a word in English, and typing out the equivalent Indonesian word.
First, make sure that you have not disabled audio in the app via the home screen audio control seen here.
If issues still persist, you can attempt to use the following Apple support link to repair your device.