Changing the alpha of a m.c. with a slider

Hello!

I have been checking out the following tutorial:
http://www.kirupa.com/developer/mx/volume_slider.htm
on how to adjust the volume with a slider.
it’s great!

what I’d like to do is use that same slider to adjust the alpha
of a movie clip.
is that possible?
or maybe adjust the alpha of a symbol.
what i’m trying to do is create “mood lighting”
where the user can move a slider and lighten or darken an interior
space, by adjusting the alpha of a dark square on top of the scene.

can anyone help me???

thanks,
jeremy.