re:
Welcome to the forums.
That's unfortunate about your job. As for the question at hand - I would say step one, rather than fumbling trying to kill the app would be to just turn the volume down. Also, upon opening the file you should have had SOME kind of interface that would allow you to pause the audio, I can't imagine it played entirely in the background. What app did you use to locate and select the file to play?
Some audio players put a persistent item in the notification shade that has playback controls for the media that is playing, that would be the next place I'd look.
Finally - to stop an app from running in the background, go into settings, application manager and find the app you think is playing. Tap it, then tap the force stop button, this should stop the app from running in the background.
I'm going to ask our mod team to move this post, this is more of a general question than an "Ask Daniel" question...