Since the audio file is a local filesystem path and thus you can't send server headers (like Content-disposition and Content-type) with it, your options to force a download are somewhat limited.
I assume you are using Chrome. Apparently, there is no way to easily disable the built-in audio player in Google Chrome but you still have a couple options to play the file without a plugin:
1. Right-click the link and choose "Save as..." from the menu. Then click on the file from the Downloads toolbar or window and it should play using whatever the default external application is for that file type
2. Use the HTML5 download attribute to force download. This will only work on browsers that fully support HTML5 - mostly newer versions of Firefox, Chrome and Opera
<a href="file:///local/path/to/file/StraightOuttaCompton.mp3" download="StraightOuttaCompton.mp3">download file</a>