I think I have found the solution

http://trac.ffmpeg.org/wiki/How%20to%20concatenate%20%28join,%20merge%29%20media%20files




2014/1/20 Alberto Megia <amegia@savemeeting.com>

> What I want is something like:
>
> cat file1.m4a file2.m4a ... fileN.m4a > file.m4a
>
> But that does not work for m4a format.
>