This extension searches the current page for links to mp3 files. When it finds them, it sets the 'download' attribute. That way, when the link is clicked, the file downloads rather than plays. This saves the effort of right-clicking and selecting "Save link as...".
Also, when the extension icon is clicked, the popup lists all of the mp3 links found as download links. That way, if the link is embedded in an "audio" tag, you can still download the file.
It does not work on sites where you are not intended for playing or downloading mp3 files. It was not intended for YouTube.
If the extension is not working for you, please include the site you tried it on in your comments.
v2.2 - Download attribute stopped working for some reason. Changed download links to audio tags with download button.