how to merge audio using ffmpeg, not concat

Solution 1:

For helping others, I'm posting the solution I found :

http://www.ffmpeg.org/ffmpeg-filters.html#amix

Mixes multiple audio inputs into a single output.

For example

ffmpeg -i INPUT1 -i INPUT2 -i INPUT3 -filter_complex amix=inputs=3:duration=first:dropout_transition=3 OUTPUT

Above command will mix 3 input audio streams to a single output with the same duration as the first input and a dropout transition time of 3 seconds.