TIL cat will correctly join MP3 files http://t.co/mmc4NN2Kmp

— Tim Hopper (@tdhopper) March 15, 2015

From Stackoverflow:

An MP3 file is nothing more than the raw MPEG2-Layer 3 (audio) stream data, there is no file level header structure with, for example, duration, original source, encoding info.

Thus

cat file1.mp3 file2.mp3 > out.mp3

can join MP3 files.