Why is there no person speaking High Valyrian?
On spanish, the is a person saying all the words, on High Valyrian, there is silence, and I don't know why.
Because there is no audio (yet). As far as I know, it's planned for when the course in complete.
The audio is not yet available. But someone made it available in memrise. https://www.memrise.com/course/1586994/duolingo-high-valyrian-audio-wip/ Learn the words and pronunciations there and sentences in duolingo.
On spanish, the is a person saying all the words
It is not a person, but a computer generated speech (3rd party library).
It is called TTS: Text-to-Speech