ffmpeg -i "concat:01.mp3|02.mp3|03.mp3|04.mp3|05.mp3|06.mp3|07.mp3|08.mp3|09.mp3|10.mp3|11.mp3|12.mp3|13.mp3|14.mp3|15.mp3|16.mp3|17.mp3|18.mp3|19.mp3|20.mp3" -acodec copy output.mp3