This language captured people's imagination and interest strictly as a spoken language, not a written one. To not make the audio component a priority seems like a great folly to me, because that it what is going to attract people. I fear, then, that the interest that would have been gathered quickly and easily will instead fall off with equal speed as people come here and discover that they aren't learning how to speak the language, only to read it. It's a proverbial shot in the foot before you even get a chance to see what it could do, like going through the work of designing a great car and then saying, "Well, they don't have engines now, but if there's enough interest then we'll start adding them." Cars are meant to be driven, and Klingon is meant to be spoken.
I understand your point, but give it time. Adding audio is not that simple, especially for such a derivative, invented language with no native speakers (sorry, but it's true). Remember, this is a beta version. The more popular it becomes, the better the chances audio will be added.
In addition, Duolingo is free at this level, and if I understand correctly, run mainly by unpaid volunteers. There are also plenty of other sources out there for learning how to pronounce Klingon correctly. Baby steps! I for one commend Duolingo for even taking on such a non-standard language. :)
Because the verb has a direct object here (the language).
Remember that verb prefixes mark not only the subject, but also the object or absense of one.
bI- indicates "subject = you [one person]; no object" while Da- indicates "subject = you [one person]; object = him/her/it/them".
In this case, the object of Hol "the language", which counts as an "it", so you need the prefix Da-.
If you want to refresh your memory of those two prefixes, have a look at https://www.duolingo.com/skill/kl/Useful-phrases/tips-and-notes .
If you didn't know of the existence of the tips and notes: they're available on the website https://www.duolingo.com/ but not (so far) on any mobile app, at least not for Klingon.
Visit the website in a browser, then click on a lesson unit and then on the lightbulb icon: