If it is all m4a, then the problem is a bit more complex: m4a contains timecodes, which may or may not start at zero. The concat demuxer is notorious for not considering this. In broadcast we allways convert to raw PCM (or something else that has not timestamps), then concat, then create the final output format.
Along the lines of
ffmpeg -i file1,m4a -c:a pcm_s16le -f s16le file1.pcm ;
ffmpeg -i file2,m4a -c:a pcm_s16le -f s16le file2.pcm ;
cat file2.pcm >> file1.pcm ; 
ffmpeg -f s16le -i file1.pcm -c:a aac -b:a whatever -ac whatever output.m4a

ffmpeg -i file1.m4a -c:a pcm_s16le -ac 2 -ar 48000 -f s16le file1.pcm
ffmpeg -i file2.m4a -c:a pcm_s16le -ac 2 -ar 48000 -f s16le file2.pcm

ffmpeg -f s16le -ac 2 -ar 48000 -i file1.pcm -c:a aac -b:a 192K -ac 2 output.m4a

• First I tell ffmpeg to open a file of format raw PCM (signed 16bit low endian -f s16le) containing 2 audio channels (-ac 2) of 48KHz sample rate (-ar 48000) and stored in file1.pcm (as usual with ffmpeg, the order of the parameters is important)
• then to reencode with AAC (-c:a) and a bandwidth of 192Kbit/s (-ba:a) again in stereo (-ac 2)

linux命令实现音频格式转换和拼接的方法

eric@ray:~$ sudo apt install FFmpeg flac

eric@ray:~$ sudo apt install lame faac

eric@ray:~/Music$ ffmpeg -i Gracie-Theme.ape Gracie-Theme.wav

##或者

eric@ray:~/Music$ flac -d Gracie-Theme.flac Gracie-Theme.wav

##使用lame命令：使用VBR动态码率压缩，0表示质量最高，9表示质量最低，默认为4

eric@ray:~/Music$ lame -v Gracie-Theme.wav

##或者使用faac编码得到m4a(mp4)、mp3格式文件：100表示质量最高 100% 

eric@ray:~/Music$ faac -w -q 100 Gracie-Theme.wav -o Gracie-Theme.mp4

#这里使用的是ffmpeg内置的aac编码起，设置码率为固定的320k

eric@ray:~/Music$ ffmpeg -i Gracie-Theme.wav -strict experimental -c:a aac -b:a 320k Gracie-Theme.m4a

#还可以使用ffmpeg将解压和压缩为一条命令:-map_metadata 表示保留歌曲元数据，即名称、演唱者等

eric@ray:~/Music$ ffmpeg -i Gracie-Theme.flac -ab 320k -map_metadata 0 Gracie-Theme.m4a

#!/bin/bash

for FILE in *.ape;
do
ffmpeg -i "$FILE" temp.wav;
lame -b 320 temp.wav "${FILE%.*}.mp3";
rm temp.wav
done

eric@ray:~/Music$mp3splt -c song.cue -o @n.@t song.mp3

#音频合并（两个音频重叠）

eric@ray:~/Music$ffmpeg -i first.mp3 -i second.mp3 -filter_complex amix=inputs=2:duration=first:dropout_transition=2 -f mp3 third.mp3

#两个音频拼接

eric@ray:~/Music$ffmpeg -i d1.mp3 -i d2.mp3 -filter_complex '[0:0] [1:0] concat=n=2:v=0:a=1 [a]' -map [a] j5.mp3

#三个音频拼接

eric@ray:~/Music$ffmpeg -i 片头.wav -i 内容.WAV -i 片尾.wav -filter_complex '[0:0] [1:0] [2:0] concat=n=3:v=0:a=1 [a]' -map [a] 合成.wav

ffmpeg64.exe -i "concat:123.mp3|124.mp3" -acodec copy output.mp3

ffmpeg64.exe -i 124.mp3 -i 123.mp3 -filter_complex amix=inputs=2:duration=first:dropout_transition=2 -f mp3 remix.mp3

ffmpeg64.exe -i 124.mp3 -vn -acodec copy -ss 00:00:00 -t 00:01:32 output.mp3

ffmpeg64.exe -i null.ape -ar 44100 -ac 2 -ab 16k -vol 50 -f mp3 null.mp3