This weakaura is already part of https://wago.io/bfaraid2 , make sure to only have one of them.
Also keep it in a dynamic group.
"Sound at x stacks" plays the specified sound whenever the specified amount of stacks is reached.
"On Application" plays the specified sound if the specified aura is applied to your co-tank. (used for auras that don't have stacks)
Showing an aura is not required for the sound to work, so if you turn off "Show Sacred Blade" for example while "Sound at x stacks" is set to 4, then sound will play but aura won't show.
The default sound requires: https://wow.curseforge.com/projects/sharedmedia_causese
Either get the addon or change default sound.