Doh! I FINALLY just figured it out. Although most of you likely already knew this, here is the answer in the event another novice (like me) has the same problem:
When you first tell it to play the audio, and you pick which audio file to play,and decide whether to loop the sound or not, at the bottom of that menu it will say:
Tag (optional): "" <-- What you need to do is NAME the sound file between the quotes. Even though the filename is "heartbeat1" you STILL need to 'tag' the filename (call it whatever you want -- but probably just easier to 'tag' it with the same filename.
Then, when you tell the program to stop playing the sound, you type in between the quotes the name that you 'tagged' it. Therefore, every single time you add a sound to your game, I suppose your should also 'tag' it with a name.
Hope that helps.