For both Steam and IW4x versions of Mw2: First, download the mod.It's very simple and easy! Just take the "images" and "sounds" folders and drag and drop or copy and paste them into your "userraw" folder in your Modern Warfare 2 directory. (where you have Mw2 install...
then open the console and type in the command /loadzone sounds from the menu when the mod has launched. Now you can join the server with the corresponding mod you have already launched and the BLACK sounds will work