I believe you would need to convert it to a wav. I have not managed to get mp3 files to work in eclipse.
Convert by opening the mp3 up in Audacity (https://www.audacityteam.org/) and then re-save it as a .AU or WAV file. (These files can get big really fast, so you may want to use audacity's editing tools to trim it to just part of the MP3 file too.)