I use a very good software for me, dBpowerAmp Music Converter. It is able to convert every sound files in any others. Very easy to use, and it can also convert a playlist m3u (this was very good for me to make an automatic conversion of all my simfiles WAV->OGG).
But the problem is that it is not a free software... And I don't know if I'm allowed to provide you a download link in tis forum (I think not... just pm me if you're interested in).
As I've said somewhere else, a pretty good program is MEDIACODER. It's freeware and it converts many audio and video formats. Plus, it gets updated very regularly.