mp3 sound - play and stop

I made to player with only the buttons play and stop, I tried to make that button toggle, but I did not works, then I made two buttons, when I click in play it shows a button and hides the other and vice versa, he functions normal, my problem is the following one:
When finishes the music the button not return for stop button, it continues showing to the animation of the play button , How can I make when to finish the music the button comes back to the state where it was.
AS
[AS]
var mp3Musica:Sound = new Sound ();
//
tocar_mc._visible = true;
parar_mc._visible = false;
//tocar música
tocar_mc.onPress = function () {
tocar_mc._visible = false;
parar_mc._visible = true;
mp3Musica.onLoad = function (sucesso:Boolean) {
if (sucesso) {
mp3Musica.start ();
}
};
// carregar música
mp3Musica.loadSound (_root.musica, true);
};
//parar musica
parar_mc.onPress = function () {
tocar_mc._visible = true;
parar_mc._visible = false;
mp3Musica.onLoad = function (sucesso:Boolean) {
if (sucesso) {
mp3Musica.stop ();
}
};
mp3Musica.loadSound (_root.musica, true);
};
//cor fundo botao
new Color (tocar_mc.fundo1_mc).setRGB (parseInt (cf, 16));
new Color (parar_mc.fundo2_mc).setRGB (parseInt (cf, 16));
[/AS]

Sorry about my English