concatenate 3 mp3 files into one m4a file
Meanings of : -filter_complex "concat=n=3:v=0:a=1" :
concat means use the media concatenate (joining) function.
n means confirm total count of input files.
v means has video? use 0 = no video, 1 = contains video.
a means has audio? use 0 = no audio, 1 = contain audio.
-f means force set file format (to see all supported formats, use ffmpeg -formats)
-vn means disable video (and also -an would disable audio if not wanted)
-y means overwrite output files (if the output file already exists).
IN CLI
ffmpeg -i input1.mp3 -i input2.mp3 -i input3.mp3 \
-filter_complex "concat=n=3:v=0:a=1" -f MOV -vn -y input.m4a