To install, navigate to your Steam folder and place the sound in the zip you downloaded in your /friends folder. Make sure that you overwrite or backup the old Steam default sound located in the same folder. Now, right-click on the file you just added and select Properties. Toward the bottom of the window that appears, check the box next to "Read-only", and then press "OK" to save your settings. Now you're done! You don't even need to restart Steam, either. How about that.