Here's one I wrote a blog post[0] about on how to merge two audio tracks of a video into one track without re-encoding the video:

    ffmpeg -i 'input.mkv' -filter_complex '[0:a:1]volume=0.1[l];[0:a:0][l]amerge=inputs=2[a]' -map '0:v:0' -map '[a]' -c:v copy -c:a libmp3lame -q:a 3 -ac 2 'output.mp4'

[0] https://blog.nytsoi.net/2017/12/31/ffmpeg-combining-audio-tr...