Thanks to the LAME Project and Pyehouse Development we now have a convenient way of creating MP3 audio files right from Quicktime Player (when you have purchase a “Pro” key).
Download both the framework and the Quicktime component from this page. Place the framework in /Library/Frameworks and the component in /Library/Quicktime. Restart Quicktime Player and you're all set to export any type of audio that Quicktime understands to MP3.
Yes, you could use iTunes to do all of this, but this allows you to cut out the middle man.