Can this also be "mara understands him/her/it"? Another example was "mara yaj", meaning "he understands mara".
As I understood, when both object and subject are 3rd person, there is no prefix. So this could be translated as "mara understands it", or is "'oH yaj mara" needed?
That's absolutely correct. When there is no prefix (or, the null prefix, as we sometimes call it) there may or may not be an object. Context will usually make it clear which is appropriate, but in cases where it is not so clear, it is probably a good idea to include a pronoun (like 'oH).
Since this exercise does not include either context or a pronoun, we accept translations with or without an object.
The Duolingo administration decided not to provide audio for the Klingon course. The Contributors and Moderators would love to add it and Duolingo is working on a method that might allow us to do it, but at this time, we do not know when or if it will be added.
There is a Skill a little bit into the course which teaches the names of the Klingon letters and has a description of the pronunciations in the Tips & Notes, however, that skill also lacks audio. If you want to check out some ideas for learning the pronunciation, check out the thread at: https://forum.duolingo.com/comment/26605745
If you want to read about the other known issues in this course, read the post at: https://forum.duolingo.com/comment/26778885
And finally, the only way I know of to complain to the administration about the lack of audio on this course is to file a bug report: https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug-