Playing a mp3 when a movie clip loads

Question

I am using Adobe Flash Professional CS6
What I am trying to do is play a mp3 file that I have in my library when the clip loads and stop playing it when the a new clip is selected from the menu I created.  On the button that loads the movie clip (Instance name: Slideshow), I have the following code:
[code]
on (release){
    gotoAndPlay(120);
onClipEvent (load){
    mySong = new Sound();
    mySong.attachSound("MastersThisHall");
    mySong.start(0,50);
onClipEvent (unload) {
    mySong.stop();
[/code]
- I went to the properties of the MastersThisHall.mp3 file in my library,
- Selected the "ActionScript", tab,
- Checked the box that says "Export for ActionScript",
- Unchecked the "Export in frame 1" check box, and
- Added MastersThisHall in the "Identifier" field.
The sound is NOT playing when I when the "Slideshow" button is pressed.
Ideas?

Answer 1

Thank you for all of this; it is very helpful.
I have series of 6 buttons I created, each of them load a different movie clip. I need to have the music from the slideshow NOT continue to play when a new movie clip is selected.
I have the following code.
Frame 1:
SlideshowMusic = new Sound(this);
SlideshowMusic.attachSound("MastersThisHall");
stop();
Frame 120 (attached to the Slideshow movie clip object):
onClipEvent (load) {
    SlideshowMusic.play();
The "Mute" Button:
on (release) {
    SlideshowMusic.setVolume(0);
The "Resume Sound" Button:
on (release) {
     SlideshowMusic.setVolume(100);
And the "Pause" Button:
on (release)
  if (SlideshowMusic.position < SlideshowMusic.duration)
    SlideshowMusic.start(SlideshowMusic.position / 1000);
It is loading and working perfectly. However, I have two more questions:
1. How do I make the the sound automatically stop when a new movie clip is played?
2. How do I change the "pause" symbol to the "play" symbol when the "Pause" button is pressed?