The introduction of Nine Songs is very simple. They are not all Yin Walkers; some gain their abilities through Nine Songs incense burners, and during peaceful times, they might not even know each other.
The inheritance method of Nine Songs is mostly an old passing to the new. It doesn't require the Witch to personally train them; they choose suitable people themselves, passing down their skills, respective incense burners, and the responsibilities of Nine Songs, just like Yao Zhen and her master.
Regarding Nine Songs, Sang Que sums it up in her own words,
The [Storyteller] is responsible for the promotion of Nine Songs. His stories can place everyone in a favorable environment when faced with powerful opponents.
