Shush the Game is a very simple and lightweight addon meant for people who use voice relay software (Ventrilo, TeamSpeak, etc.)
With Shush the Game, when you hold your Push To Talk (PTT) key, the game sound will lower itself while you speak. When you release the key, it will return to its previous volume.
To use, install the addon and check your keybindings menu. You will see a new category called "Shush the Game" and a single binding for your PTT key. Change this binding in-game to the same binding you use for your voice relay software, save the bindings, and you're set.
Note: some bindings commonly used for voice relay software cannot be used as keybinds within WoW. There is no way around this; you'll have to remap your keybind for PTT if you wish to use this addon.