Buttons to control sound volume HELP

programming flash
1

I have to buttons that control the volume. They work fine just that I want to stop when the volume hit 100 and also when you go down to stop at 0. Right now it will go up and down then start the opposite way. I am not quite sure how to stop that(with a if statement).Could use help.

var upVolume:Number;
upVolume =30;

this.up_ro.onPress = function() {
song.setVolume(upVolume += 5);
up_mc.gotoAndPlay(“rollout”);
up_an.gotoAndPlay(“rollout”);
};

this.down_ro.onPress = function() {
song.setVolume(upVolume -=5);
down_mc.gotoAndPlay(“rollout”);
down_an.gotoAndPlay(“rollout”);
};