'oH is a pronoun: "it".
machbe' on its own could mean any of "he is not small - she is not small - it is not small - they are not small".
Adding the subject 'oH "it" means that machbe' 'oH can only mean "it is not small".
machbe'. tIn. would also be completely fine grammatically, but a bit more ambiguous.
The Duolingo administration decided not to provide audio for the Klingon course. The Contributors and Moderators would love to add it, but at this time, we do not expect it to 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-