M4A is a weird format, so you have to be creative here is a quick copy of what I did.
Basically you need to convert to .wav to make use of them and thenI converted the resulting .wav into an mp3 (nice small file size and basically universally playable):
=======
sudo apt-get install mpg321 mp3gain faad normalize lame
faad "Voice 002 (copy).m4a"
faad "Voice 002 (copy).m4a"........