BRAVO!!! I really like your adventure! And in order to help you in acquiring links, I think you should start at the very beginning.
Where did this Slaneesh cult originate from and who exactly is in it?
I would say that for this adventure, with so many people having to go missing and getting butchered, that everyone in that Monastery is a disciple of Slaneesh (to some extent or less). They might not all be hard core fanatics cutting people up and using their body parts, but they contributed something either by supplying their vocal cords/body parts and/or by staying silent. Heck, you could even have one unwilling person with eyes, tongue, and vocal cord missing, who is trying to warn the PC but is unable to because he is considered "old and senile" and has a "care taker."
Anyhoot, lets say they are all part of the cult, with some more than others. Lets say they all became this way because an artifact that was brought to them containing the message of the Slaneesh daemon. One weak willed person became corrupted, who then showed another weak willed person the "light", and so forth and so forth. Where did this artifact come from? Well, that is for another adventure seed. It could of been planted/sent there by the Governor, another Priest, sold to them by a passing merchant, donated as a gift, or mysteriously turned up on their door steps.
Next is the mission. You already know what you want accomplished and resources needed to accomplish it. A musical organ made out of human body parts. So clues can lead to that.
Maybe a priest is seen with his hands all bloody after one of the NPC's go missing. Maybe a room is discovered with blood all in it, a drainage pipe with hair and skin and the smell of iron coming out of it, etc etc. I mean heck, what do the priests do with the rest of the body parts? They don't eat it, that's for sure. Maybe they burn the extra body parts and/or the clothes, and the PC discover the remnants of a belt buckle, comb, thigh bone, etc in the incinerator. Or maybe they use the extra body parts as fertilizer after grounding them up, or for farm animal food, etc. And the PC discovers a finger bone sticking out of the soil in the garden out back that he isn't allowed to go to, or in a pigs mouth, etc etc.
Maybe one of the cultist priests is writing stuff on his bedroom wall in his own blood, when the PC discovers it. Like the PC is walking by the door and he hears whimpering , giggling, whispering coming a room. Upon opening it he sees a priest drawing on the walls with his own blood. The symbols the priest is drawing hurt the PC's eyes to look at, but there is also a message written in the blood. You can have this say "_____ coming is near" (with ____ being the name of your daemon) or anything else you desire. And once the PC reads this, the priest notices him, draws a dagger from his robes, and starts to attack the PC. The fight ends up in the hallway, with other priests coming to stop the fight and drag away the "sick" priest and blockading the room from anyone's entrance besides the Abbott. All the while the PC might be asking the priest to stop attacking him, and all the "crazy" priest can do is moan because he has no tongue or vocal cords (all the PC sees is a scar along his neck and no tongue). Later, you can ask the PC why did he hear whispering when the guy had no tongue? Enter mysterious music.
It could be that in some of the other priests journals that they write, they mention "The rights of passage" and the "removal of unjoyous sounds." These journals (not all the Priests/Cultist should have them, only a few), could have other details too. Like "Brother Liam fought like a mad man. Why did he not want to be part of the _____ coming/resurrection?" And Brother Liam could be a fellow priest gone missing without a trace, or possibly one of the band members.
Anyhoot, I think you get the jist. I do recommend though that you include an NPC that you control to be with the PC and helping him out from time to time. Maybe you are going to introduce a few NPC's later after he becomes part of the Inquisition, but you can add one now too if you like. At the end this, the NPC could also be recruited by the Inquisition but is sent to a different team. And maybe later your PC runs into him, but the NPC is now secretly a bad guy. Best of luck to you though no matter what you decide.
P.S. Did I mention that I really like your adventure?