Using the simple copy command line code , we can easily able to join two or multiple .mp3 audio files quickly and good thing is that all process will be done without any additional programs.

Place two mp3 audio files in the same folder. In my case i have added two mp3 (which i will use to merge together) files on my desktop.
Files names are 1.mp3 and 2.mp3

For this we will use following command to combine both mp3 files

copy /b 1.mp3 + 2.mp3 new.mp3
This was the process screen.
1 file(s) copied.
At the desktop i can see the new.mp3 file that contains both files audio together.

For mac or linux OS we can use following command in Terminal

cat 1.mp3 2.mp3 > new.mp3
Thats it.