This Addon plays Illidan's "We Now Hide in the Shadows, Hidden From Our Enemies!" upon using Shroud of Concealment.
It is set to play with Master sound, so it will still fire and be heard, even if you keep your "Sounds" turned off in game, as I sometimes do. As long as your "Master Sound" is set high enough, it will still be loud enough to hear.
I am also working on a couple other projects at the moment, so keep your eyes peeled.