I am not a Mac expert but consider a scenario where you are using a Mac and someone send you a music in windows media audio(.wma format) or any other format, how are you supposed to convert it to mp3?
First off, there is a quick solution if you are using VLC Media Player. Open it, go to Media > Convert/Save, add any music file, and hit Convert/Save button. Now enter the destination folder and file complete with .mp3 extension, finally select Audio(MP3) from Settings and hit Start. You will now get an mp3 audio file.
Another method is to use a free tool called All2MP3 which can convert audio formats such as, APE, MPC, FLAC, WV, OGG, WMA, AIFF, and WAV to MP3 format.
You just need to add the music files, select the quality and hit Convert.
Screenshot above is taken from MacUpdate.
This tool was reported as quite buggy but the latest version is said to be stable.
It works on Mac OS X 10.4 and later. Enjoy!