How to achieve volume slider effect like on vimeo in flash?

Question

The video player (aka moogaloop) on vimeo.com has a neat visual effect for the audio volume slider. There are a number of sound bars that rise and fall when the mouse hovers over them. The volume level is shown by a blue filled area under the bars with pixel precision.

This puzzles me, how can the multiple masks (bars) be used to cover a single blue rectangle?

Update: Masks in flash: "the mask item acts as a window that reveals the area of linked layers beneath it [...] A mask layer can contain only one mask item". How do I get multiple masks to clip over one layer?

Answer 1

Put your moving/interactive bars inside one movieclip that is in a layer masking your blue box, and you're done!