Wow, almost three years and 750 posts on this site, and you mean you've never actually opened a file in an audio editor, done some basic editing, and exported it to mp3?
I use Audacity
because it's a kick*ss audio editor that's free. And it should in no way be so hard that you need to have someone else do it for you
Oh, by the way, if you want to get below the 1.5 MB (?) file size limit, I would advise that you either make a shorter clip (my favorite method!) or that you use a very low mp3 bitrate (like 64kbps mono or lower).