I recently got a bunch of m4a files from a mac user from her band...I know I can play it with amarok but not xmms. Also why not just use mp3s which you know everyone and their grandma can use. So below is a quick bash script to automate the process. I'm sure there is a quicker way...but you can use this to convert to mp3s or ogg vorbis or whatever format you like.

Code:
#!/bin/bash
#
# Dump m4a to wav (first step in conversion)

y=`pwd`
cd "$1"

echo changing directory to $1
sleep 15
echo converting the damned .m4as to mp3s
for i in *.m4a
do
mplayer -ao pcm "$i" -aofile "$i.wav"
done

echo converting the damned .wavs to mp3s
for i in *.wav
do
lame -h -b 192 "$i" "$i.mp3"
done

echo renameding the damned mp3s
for i in *.mp3
do
x=`echo "$i"|sed -e 's/m4a.wav.mp3/mp3/'`
mv "$i" "$x"
done

rm *.wav
rm *.m4a
cd $y

echo changing back to $y