Convert audios on Mac

Nowadays, on the Internet, there are really diverse formats of audio files. I often
find the need to convert files from one format to another. For naive users, this
can be a challenging task. But actually it can be a trivial script or set of commands
if you know how to tap the power of all the marvelous open source tools, esp.
mplayer and lame, etc.

These two links show the basic example.
http://www.linuxquestions.org/questions/linux-general-1/converting-m4a-to-mp3-170553/
http://gimpel.gi.funpic.de/wiki/index.php?title=Howto:convert_aac/mp4_to_wav/mp3/ogg_on_Linux

You can also convert other formats, like flash, too, as long as you have all the plugins.
The Mac specific build of MPlayer is here:
http://mplayerosxext.googlecode.com/files/MPlayer-OSX-Extended_rev13.zip