I want audio files to be merged and be the length of the longest one of course.
Not to concat them, which means adding one behind the other.
I tried googling since yesterday night, but didn't found any such thing. Is this really possible ?
For helping others, I'm posting the solution I found :
Mixes multiple audio inputs into a single output.
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.