here is how to fix this infamous bug.
If you have charisma 16+ and you sing at each menhir you have no bug, harper sound and ghost in the end.
But if you want the trophy you have to "bad singing" using a character with low charisma to have the riddle.
And here is the catch, you can only bad sing with this character (m'khiin is ok) at 4 menhirs, each menhir is active after harper sound. There are 2 menhirs which no give harper sound with low charisma and bug the quest, these you have to active with high charisma character.
Here what i did: first menhir that start quest, high charisma only, low will bug quest and no harper sound.
Then go to the 2 east menhirs, then the menhir up the campside, all with bad singing.
Go to pit menhir (where baeloth was) , use high charisma, low will bug the quest.
At least go to the statue menhir , use low charisma, answer riddle , and the ghost appear.
Just remember that menhir are well activated after harper sound and there is no way to reset quest, restore a former savegame