If you have some sound or music files to convert the format, I think the SoundConverter is a good choice.
To use the SoundConverter in the command line, here is an example to convert *.ogg to *.mp3.
(Why bother to convert ogg to mp3? Because I have an old music player which doesn't support ogg format! :-p )
Suppose you are in the folder containing ogg audio files:
$ soundconverter -b -m audio/mpeg -s .mp3 *.ogg