Automate Conversion from m4a to mp3?
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.