FFmpeg

echo 'apt-get update;echo;apt list -a --upgradable;apt-get upgrade -y'|sudo bash
echo 'apt-get install -y ffmpeg'|sudo bash

Convert » WebM » MP4

for webm in *.webm;do ffmpeg -v error -i "${webm}" -qscale 0 "${webm%.*}.mp4"; done
for webm in *.webm;do ffmpeg -v error -i "${webm}" "${webm%.*}.mp4"; done

Convert » MP4 » MP3

for mp4  in *.mp4; do ffmpeg -v error -i "${mp4}"  "${mp4%.*}.mp3";  done

Playground

mkdir -p ~/Documents/ffmpeg-playground cd ~/Documents/ffmpeg-playground	for webm in *.webm;do echo ${webm%.*}; done for webm in *.webm;do echo ${webm##*.};done	ffmpeg -hide_banner -loglevel error -h ffmpeg -v error -h
for webm in *.webm;do ffmpeg -v error -i "${webm}" -qscale 0 "${webm%.*}.mp4"; done for webm in *.webm;do ffmpeg -v error -i "${webm}" "${webm%.*}.mp4"; done for mp4 in *.mp4; do ffmpeg -v error -i "${mp4}" "${mp4%.*}.mp3"; done

References

FFmpeg » Ubuntu » Convert WebM to other formats FFmpeg » Ubuntu » Quieter/Less Verbose FFmpeg
Linux » Bash » Extract Filename & Extension