I have never tried to replace a game sound. I have only done custom music. I don't know if sounds would use the same protocol, but here's how it works for music if you want to experiment.
You can replace any piece of music in the game by creating a folder called 'custom music' in the game directory. You then find the sound you want to replace and note the full directory path. Create that same nest of folders in 'custom music' and put the replacement music file there. Be sure to give it exactly the same name as the original. The game client apparently always checks to see if there is custom music before loading the default. I used this trick to play the Freeport GH music in our Qeynos GH.
As I say, I don't know if this works for sounds, but it may be worth a try. Since anything you put into the regular sounds folder will be replaced with the next patch, it seems reasonable to try making a 'custom sounds' folder.
-= Therendil =-