Hi guys, just throwing this here in case you are also annoyed with that GlobalCooldown sound (not the error speech that can be disable in system>sound) so I found this WoW addon tweak that works perfectly!
Here it is:
Basically since 8.2 patch we cannot replace sounds anymore, so all those old addons like SoundBeGone and such are not working.
This one works out of the box and you do not need to tweak anything.
Hope this helps someone as it helped me a lot
PS: See custom.lua for muting/unmuting custom sounds