I use Foobar2000 for that (and everything else), it takes a bit effort to learn it though.
Converting should* be as easy as opening the files in foobar2000 and right click and Convert. Then you can select the output format
* You will need to download the mp3 encoder seperatly. You can do that here: http://www.rarewares.org/mp3-lame-bundle.php . Unpack the lame.exe in a folder somewhere, Foobar2000 will ask for this file when you convert to mp3.
The reason most apps are payware is that they have to pay for a license to the creators of MP3. Thats also why Foobar2000 doesnt do it out of the box, you download the (free) encoder seperately. But you don't have an actual 'license' to use it.
Tried Wordfeud and didn't like it. Unable to see how much a word is worth before submitting it and you can't recall all your tiles if your word isn't valid or doesn't fit. You have to drag each tile back one at a time. Sticking with Words By Post.