WowAce

So I have noticed that Decursive's audio trigger is linked to the Sounds Slider in the sound settings menu. Is there any way to lower Decursive audio trigger while not having to adjust in game sounds as a whole? Because it is way too loud. 

hmmm I'm not sure about that, check this add-on: https://www.curseforge.com/wow/addons/volumizer it allows a more granular control of the different volumes.

I remember why it's like this, it plays on the master channel so that you can set other volumes low (SFX, Music, Dialog, ambiance...) and still here Decursive's alert. In your case, what you need to do is to augment other volumes so that they are closer to your master volume and then lower your master volume.