You can remove the sound for the notifications like this:
Code:
/script SOUNDS["NEW_NOTIFICATION"] = SOUNDS["NONE"]
Until reloadui the sound for the notifications cannot be used anywhere in the game anymore as it simply will try to play the "None" sound then -> Which is silent.
You can also use any of your addons and edit the main lua file of it with a text editor.
Search for "EVENT_ADD_ON_LOADED" and at the end in this line is the callback function which is run as this addon will be loaded.
Search this callback function in the addon file(s) and at the end of the callbackfunction add this then:
Code:
SOUNDS["NEW_NOTIFICATION"] = SOUNDS["NONE"]
This will make THIS addon exchange the sound for you as this addon loades and you do not need to use the script anymore each time.
You could maybe add it to changes addon event EVENT_ADD_ON_LOADED callbackfunction, which is in file Changes.lua,
line 1004 (in function onAddonLoaded):
Add the line here:
SOUNDS["NEW_NOTIFICATION"] = SOUNDS["NONE"]
Save, reloadui, you are set.