When things go wrong your video is basically unplayable or the first video plays fine and then freezes when moving on to the next. Generally if both videos weren't produced with the exact 100% same settings you will have issues. You can try the basic concat but it often won't work right.
Solution for me:
My example uses 3 videos in total so "n=3" and a=1 to include audio.
ffmpeg -threads 12 -i file1.mp4 -........
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"........