Add option to prevent transcoding of audio files

Checklist

• I’m reporting a feature request
• I’ve verified that I’m running youtube-dl version 2021.01.24.1
• I’ve searched the bugtracker for similar feature requests including closed ones

Description

Downloading a lossless audio file (tested with .wav and .aiff) with the -x flag causes the audio to be transcoded into mp3. For archiving purposes, having the option to prevent all (audio) transcoding would be ideal.

Examples links: https://soundcloud.com/caleesi-sarahkreis/caleesi-sarahkreis-mbr-sao-paulo https://soundcloud.com/caleesi/arutani-caleesi-ennui-original-mix

[pukkandan]

I looked through the source code of extractaudio. These are the conversions it does when you don’t give --audio-format:

aac > m4a flac, mp3, opus = untouched vorbis > ogg everything else > mp3

[inorick]

As @pukkandan wrote, -x flag always transcodes/converts audio;

This is not completely true as using -x on an .mp3 stream will not cause transcoding:

youtube-dl.exe -x https://soundcloud.com/talderverwirrung/caleesi-im-tal-057
[soundcloud] talderverwirrung/caleesi-im-tal-057: Downloading info JSON
[...]
[download] Destination: Caleesi Im Tal - #057-956484631.mp3
[download] 100% of 56.93MiB in 00:04
[ffmpeg] Post-process file Caleesi Im Tal - #057-956484631.mp3 exists, skipping

It looks like the current rule of the flag is “strip video and transcode audio unless it is mp3”.

The current behavior makes it harder to use youtube-dl in a script to extract audio (with minimal transcoding) from multiple sources (e.g. soundcloud + youtube). It would require to set the -x flag for video sites but omit it for audio-only sites.

Having a demux option looks like a way to address this. I would even argue that it should be the default since the name of the --extract-audio flag suggests extraction only. Transcoding should be an optional second step as it decreases quality further.