There are a few online programs that'll do it. The one I usually use is
http://media-convert.com/.
I was trying to use it this week to convert a file for my level, but it's not been working, so I used
http://www.zamzar.com/ instead, which works well, only you have to give it your email address, which might mean they sign you up for spam; I don't really know. I used an old email account which I always use for websites I don't trust not to sign me up for stuff.
They're both pretty simple to use, so hopefully you'll figure it out.
