Why is ytdl still not downloading audio?

[Zachary24]

Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2018.12.17. If it's not, read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2018.12.17

Before submitting an issue make sure you have:

• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Checked that provided video/audio/playlist URLs (if any) are alive and playable in a browser

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
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Description of your issue, suggested solution and other information

best downloads both audio and video, muxes them in mp4, however, rather than downloading the BEST quality, it chooses to get a 640x360 copy. Which doesn't make any sense, but okay. 137+140 works perfectly, for videos that qualify, when downloading playlists with 2000-3000 videos it's practically impossible to match that, on the other hand the standard bestvideo+bestaudio/best get's the highest quality, yet ignores [ext=mp4] and continues to mux it into an mkv. On the otherhand, the ideal 137/248/136+140/best gets audio for very few and doesn't get audio for most.

137/248/136+140/best Gets the audio for (youtube) videos like: HGobiQPPbAA , zu_MjWjT45Q ,

While it doesn't get the audio for: T4FAg5A4wQk , MTY0A8tmPO0 , 9AQIm4fEJUM

Some renditions of 137/248/136+140/best i've tried:
137/248/136+140
137/248/136+140/best/best
137/248/136/bestvideo+140/bestaudio
137/248/136/bestvideo+140/bestaudio/best

The documentation is very unclear on why 137/248+140 gets 248 ONLY if 137 isn't available + 140, and why bestvideo+bestaudio/best gets bestvideo + bestaudio OR the best version, as there both the exact same formatting, just different media types: (video + audio / next audio) and
(video + audio / nextvideoANDaudio) if that makes any sense

[dstftw]

Bother to at least read FAQ.