Button within a Movieclip

This is a pretty basic question, but I don't understand how Flash handles button functions within Movie Clips that also have functions. Here is what I have...

A Movie Clip named controlBar_mc with a function like so:
controlBar_mc.onRollOver = function () {
showController();
}

controlBar_mc.onRollOut = function () {
hideController();
}

OK... now within this Movie Clip, I have a Button named play_btn with this code:
controlBar_mc.play_btn.onRelease = function () {
ns.pause ();
}

The problem is that the Movie Clip onRollOver function is overriding the Button onRelease function. How do I get around this?

Thanks,
g