If you have a warning in this ffmpeg code:
ffmpeg -i input.mp4 -c:a libvorbis -q:a 2 -ac 2 -af 'volume=4' -c:v copy output.mkv
Error
Non-monotonous DTS in output stream
Solvo
add '-async 1':
ffmpeg -i input.mp4 -c:a libvorbis -q:a 2 -ac 2 -af 'volume=4' -c:v copy -async 1 output.mkv