how to convert mkv to mp4

so you've recorded a video on linux with guvcview and now you want to play it/edit it on an iphone,
heres a one liner you can use with ffmpeg to convert it to mp4:

for f in *.mkv; do ffmpeg -i "$f" -c:v libx264 -profile:v high -level 4.1 -c:a aac -movflags +faststart "${f%.mkv}.mp4"; done

joining mp3s in the term

mp3 file comcatenation
this article explains a few other ways to do it. alternative to sox like ffmpeg and in great detail.