Volume control (using setVolume)

I have this script placed at the root level:

backgroundAudio=new Sound();
backgroundAudio.attachSound(“movieAudio”);
backgroundAudioVolume=40;
_root.backgroundAudio.setVolume(backgroundAudioVolume);
_root.backgroundAudio.start(0,99999);

and this script placed on a volume up button:

on (release) {
backgroundAudioVolume++;
_root.backgroundAudio.setVolume(backgroundAudioVolume);
}

It works, but there’s one problem, when the volume up button is released and the volume increases by one unit, it does so from the beginning, resetting the backgroundAudioVolume to 0 and then increasing. How can I avoid this?