ffmpeg -i LostInTranslation.mkv -vcodec copy -acodec copy LostInTranslation.mp4

The same in a loop:

for i in *mkv; do ffmpeg -i $i -vcodec copy -acodec copy $i.mp4; done

sudo apt install youtube-dl

youtube-dl --ignore-config -o '~/Music/%(uploader)s - %(title)s.%(ext)s' --extract-audio --audio-format mp3 --audio-quality 5 **// <youtube code or URL> //**

Config file: .config/youtube-dl/config

# Lines starting with # are comments

-x          # Always extract audio
-k          # Keep the video file on disk after the post- processing; the video is erased by default
--no-mtime  # Do not copy the mtime

# Do not download the DASH manifests and related data on YouTube videos
--youtube-skip-dash-manifest

# Save all videos under Movies directory in your home directory
-o '~/Videos/%(uploader)s - %(title)s.%(ext)s'

#--audio-format mp3

# Specify ffmpeg/avconv audio quality, insert a value between 0 (better) and 9 (worse) for VBR or a specific bitrate like 128K (default 5)
--audio-quality 5

# Download best mp4 format available or any other best if no mp4 available
# -f 'bestuploadervideo[ext=mp4]+bestaudio[ext=m4a]/best[ext=mp4]/best'
# Download best format available but not better that 480p
# -f 'bestvideo[height<=480]+bestaudio/best[height<=480]'
# Download best video only format but no bigger than 50 MB
# -f 'best[filesize<50M]'
# Download best format available via direct link over HTTP/HTTPS protocol
# -f '(bestvideo+bestaudio/best)[protocol^=http]'
# Download the best video format and the best audio format without merging them
#-f 'bestvideo,bestaudio'

-f 'bestvideo[ext=mp4]+bestaudio[ext=m4a]'
--merge-output-format mp4

# Bypass geographic restriction via faking X-Forwarded-For HTTP header (experimental)
--geo-bypass   

Using the preceding config :

youtube-dl <youtube code or URL>

Without using a config file :

youtube-dl youtube-dl --ignore-config -x -k --no-mtime --youtube-skip-dash-manifest -o '~/Videos/%(uploader)s - %(title)s.%(ext)s' --audio-quality 5 -f 'bestvideo[ext=mp4]+bestaudio[ext=m4a]' --merge-output-format mp4 --geo-bypass **// <youtube code or URL> //**

for f in *.flac; do flac -cd “$f” | lame -b 128 - “${f%.*}”.mp3; done