I have used this editor several times. All these editors have a bit of a steep learning curve but are powerful enough to trim and fix many problems with your music:
http://www.audacityteam.org/
It will also convert to mp3 IF you download the addon (that is necessary for patent reasons) it only takes a few seconds.
For example, I trimmed off the long held note at the end and then did a quick fade out and now the music loops better.
