How do I mix multiple audio tracks mit FFMPEG and adjust each volume?

Question

Let's say I have an input .mp4 file that contains 4 audio tracks. How can I change their volumes independently and convert it to a new file that just contains all the 4 audio tracks mixed together and stored in the first audio track? For example I want the first, second and third audio tracks from the input file to be double their original volume and the fourth to be half its original volume, all saved in the output files first audio track. How would that command look like?

Answer 1

Here you can find many good answers: How to overlay/downmix two audio files using ffmpeg
where the most comprehensive one links to https://trac.ffmpeg.org/wiki/AudioChannelManipulation

I recently had a similar use case: freely mixing 6 mono tracks of a multi-track recording to stereo output with different volumes on either or both output channels, which can be achieved like this:

ffmpeg -i 0.flac -i 1.flac -i 2.flac -i 3.flac -i 4.flac -i 5.flac \
-filter_complex [0:a][1:a][2:a][3:a][4:a][5:a]amerge=inputs=6,pan=stereo|c0=c0+1.2*c1+1.2*c2+1.3*c3+c4|c1=c0+1.3*c3+c4+0.8*c5[a] \
-map [a] output.flac