Sound won't stop

I have a background sound that I can get to play but no matter what I do, I can’t get it to stop. Basically, I have a layer called “musicbox” for the sound. On frame 20 of the main timeline, I put the sound in and in the properties panel at the bottom, set it to “start” and “loop”. There are some movies that play at frame 20 and 30 so I had to set the sound to loop because I don’t know how long it will take people to look at the movies there.
Anyway, on frame 40 I want the musicbox sound to stop and another sound to start playing. On frame 39 (musicbox layer) of the main timeline I inserted a new keyframe and marked the sound to “stop”. The only it doesn’t stop. ???
Any clues as to why it won’t stop? I’m I supposed to be naming it something? Is there a better way of going about this? I can post a link to the swf file if you need to take a listen.

Thanks!
Connie