Basic AS help for volume value check

Hello,
I have the main movie with a radio mc on stage containing an empty container mc into which I load external swf music file and 2 buttons to control volume, a ‘+’ and a ‘-’.
This is AS I’m using
in the first keyframe of the radio mc:

_root.radio.container = new Sound();

for the ‘-’ button:

on (release) {
_root.radio.container.setVolume(_root.radio.container.getVolume() - 20)
}

Same thing for the ‘+’ button, changing -20 to +20.

Now if you click madly on the ‘-’ button, the volume will do down then arrived to the point of no volume it will start going up again which is no good. With the ‘+’ button the volume will keep going up to the point of blowing up my speakers (which are already crap to start with), no good either.:stuck_out_tongue:
So I would like to add some AS so that I can set a minimum and maximum volume limit but I’m still no good in understanding on how to do that.:sigh: