Automate Conversion from m4a to mp3? I have hundreds of sound effects - clips just a few seconds long each - that are all m4a files.

I need to convert them to mp3 files. What's the best way of doing this? I see Automator can automate some conversions, but m4a to mp3 isn't one of them.

Is there a good free program or command line utility or something I can use or this? Is there maybe some way I can get QuickTime or iTunes to quickly do this conversion for me?

I can load an NSSound using an m4a... I don't see any way to get NSSound to spit out a file, though...
You could install ffmpeg, a command line tool for audio/video conversion, and then create an Automator service invoking a shell command.

For installation of ffmpeg please refer to their website. Once you have installed it, find out the location of it with this command:

$ which ffmpeg
/usr/local/bin/ffmpeg


Then, open Automator, create a new Service, change the dropdown menu "Service receives selected" to "files or folders" in "Finder", then add the action "Run Shell Script". Select Pass input "as arguments" and in the textbox, write

for f in "$@"
do
    /usr/local/bin/ffmpeg -i "$f" -acodec libmp3lame -ab 128k    "${f%.m4a}.mp3"
done  


where you replace /usr/local/bin/ffmpeg with the path to ffmpeg on your computer.
That's it!

Now, you can open the Finder, go to a directory containing *.m4a files, select files you want to convert, right-click, go to Services and select your newly created service. There will be no progress bar shown, but ffmpeg will convert your files one after another.