The "Sounds 1" / "Pronunciation 1" unit has been modified
The tree has a "Sounds 1" unit fairly early on, which was intended to give learners lots of opportunities to listen to the various sounds of Klingon, both at the beginning and the end of a syllable.
Since Duolingo decided to launch the course to beta without audio, it can't fulfil its originally envisioned purpose and had become merely a collection of odd vocabulary ("water pipe"?) that was presented without sentence context as individual words, more confusing than useful.
We can't delete the unit, but I've deleted most of the translation exercises -- I had to leave one for each of the sounds, though.
So now this course teaches the names of the Klingon letters. Just as you learned that the English letter H is called "aitch" (or "haitch"), so you can learn that the Klingon letter H is called Hay.
The letter names will not be used in the remainder of the course, so you don't necessarily need to remember them later.
For more information, including how the letter names are formed and how to translate them in the exercises, please read the tips and notes for the unit: https://www.duolingo.com/skill/kl/Pronunciation-1/tips-and-notes
Those tips and notes also give some general information on the pronunciation of Klingon which you may find useful.