Make a list of the files to be concatenated in this format:
file "chapter 1.mp3" file "chapter 2.mp3"
Then run FFMPEG:
ffmpeg -f concat -i list.txt
If everything is the same codec/sample rate etc you can use `-c:a copy` to avoid re-encoding.
Reminder that FFMPEG is retarded when it comes to non-MPEG formats; if storing in ogg/opus/flac its best to have ffmpeg output a wav file and use official xiph tools.