Overall volume control with slider

I would like to have a volume control for the overall volume of the playback and would like to have a slider for it at the top. That way, one could easily control the volume of the playback without having it affect the volume of the entire computer. (I like to have my playback fairly loud, but when my family is home, I need to turn it down. This sounds kind of trivial, but it is actually an important aspect of my working environment.)