Ffmpeg

To make whatsapp accept video (copied from reddit):

 ffmpeg -i broken.mp4 -c:v libx264 -profile:v baseline -level 3.0 -pix_fmt yuv420p working.mp4

To convert audio from .mkv to .mp3 (selecting time from 1.5s to 150s):

   ffmpeg  -i input.mkv -ss 1.5 -to 150 -vn -c:a libmp3lame -q:a 2 output.mp3