From this clip: http://www.youtube.com/watch?v=tGcsYK-e15E
Homer is singing a song....in it he says hey...now you can hear it every time one of your steam friends goes online.
Place the sound file into your program files \ steam \ friends folder(s). If you do not have a folder, make one. If the sound is not working try right-clicking it and select properties, then check off read-only.