How do I make a slider bar that adjusts the volume of music?

I'm just doing the Colour Switch game tutorial for my programming class and I want to have the volume controlled by a slider bar. How do I go about doing this?

See this demo:

app.box.com/s/le6a9ceqnlphncofqavvrp0aurtpg0nk

I see you on literally every topic in this forum and you, my friend, are a saint.

So, I tried doing what was shown in the file above, adjusting what I needed to in order to fit my program, but every time I start the layout, the slider goes to the center of the sliding bar and won't move if you try to drag it. The text also displays "NaN %".

Check settings on the slider bar, instance variables and everything else. You probably missed some small detail.