ffmpeg -i file1.m4a -acodec copy file1.aac
ffmpeg -i file2.m4a -acodec copy file2.aac
ffmpeg -i "concat:file1.aac|file2.aac" -c copy result.aac
ffmpeg -i result.aac -acodec copy -bsf:a aac_adtstoasc filenew.m4a
ffmpeg -i file1.m4a -acodec copy file1.aac
ffmpeg -i file2.m4a -acodec copy file2.aac
ffmpeg -i "concat:file1.aac|file2.aac" -c copy result.aac
ffmpeg -i result.aac -acodec copy -bsf:a aac_adtstoasc filenew.m4a