Playing a sound in a Firefox add-on

I would like to create a simple add-on that would play a different MP3 recording every time the user double clicks a word in a webpage he is visiting and selects a special option from the context menu.

The MP3 files are located on a remote server. Normally I would use JavaScript+Flash to play the MP3 file. In a Firefox add-on, however, I'm unable to load external scripts for some reason (playing the sound works fine if it's the webpage itself that loads the scripts, but of course I need it to work with every website and not just the ones that include the script).

So what's the easiest way to play a remote MP3 file in a Firefox add-on using JavaScript?