Feat Req: D/L format - best video + extracted audio track

[edrozenberg]

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Description of your issue, suggested solution and other information

Lots (maybe most?) of Youtube videos "these days" provide best video as a separate format item, but best audio is muxed with not-best video. For example below, best video is provided by format 248 or 137. But best audio is muxed as part of format 22.

Would be great if there was a way to request best overall format for mp4, and youtube-dl would extract mp4 audio from format 22 and mux it with best video format 137 (for this example). Currently best does 137+251 which is NOT the best possible audio and results in a .mkv (fair enough) which is not the mp4 I typically want.

https://www.youtube.com/watch?v=pB_V7cwbyY8

[youtube] pB_V7cwbyY8: Downloading webpage
[youtube] pB_V7cwbyY8: Downloading video info webpage
[youtube] pB_V7cwbyY8: Extracting video information
[info] Available formats for pB_V7cwbyY8:
format code  extension  resolution note
249          webm       audio only DASH audio   56k , opus @ 50k, 1.27MiB
250          webm       audio only DASH audio   79k , opus @ 70k, 1.74MiB
140          m4a        audio only DASH audio  128k , m4a_dash container, mp4a.40.2@128k, 3.02MiB
171          webm       audio only DASH audio  135k , vorbis@128k, 2.97MiB
251          webm       audio only DASH audio  152k , opus @160k, 3.38MiB
278          webm       256x144    144p  102k , webm container, vp9, 12fps, video only, 2.20MiB
160          mp4        256x144    144p  113k , avc1.4d400c, 12fps, video only, 2.60MiB
242          webm       426x240    240p  219k , vp9, 24fps, video only, 3.82MiB
133          mp4        426x240    240p  251k , avc1.4d4015, 24fps, video only, 5.82MiB
134          mp4        640x360    360p  319k , avc1.4d401e, 24fps, video only, 6.05MiB
243          webm       640x360    360p  370k , vp9, 24fps, video only, 6.57MiB
244          webm       854x480    480p  581k , vp9, 24fps, video only, 10.20MiB
135          mp4        854x480    480p  587k , avc1.4d401e, 24fps, video only, 12.14MiB
247          webm       1280x720   720p 1005k , vp9, 24fps, video only, 18.51MiB
136          mp4        1280x720   720p 1105k , avc1.4d401f, 24fps, video only, 23.98MiB
248          webm       1920x1080  1080p 1860k , vp9, 24fps, video only, 34.94MiB
137          mp4        1920x1080  1080p 2131k , avc1.640028, 24fps, video only, 45.91MiB
17           3gp        176x144    small , mp4v.20.3, mp4a.40.2@ 24k
36           3gp        320x180    small , mp4v.20.3, mp4a.40.2
43           webm       640x360    medium , vp8.0, vorbis@128k
18           mp4        640x360    medium , avc1.42001E, mp4a.40.2@ 96k
22           mp4        1280x720   hd720 , avc1.64001F, mp4a.40.2@192k (best)

[youtube] pB_V7cwbyY8: Downloading webpage
[youtube] pB_V7cwbyY8: Downloading video info webpage
[youtube] pB_V7cwbyY8: Extracting video information
WARNING: Requested formats are incompatible for merge and will be merged into mkv.
[download] Destination: Happy Hippie Presents - 50 Ways to Leave Your Lover (Performed by Miley Cyrus)-pB_V7cwbyY8.f137.mp4
[download] 100% of 45.91MiB in 01:00
[download] Destination: Happy Hippie Presents - 50 Ways to Leave Your Lover (Performed by Miley Cyrus)-pB_V7cwbyY8.f251.webm
[download] 100% of 3.38MiB in 00:04
[ffmpeg] Merging formats into "Happy Hippie Presents - 50 Ways to Leave Your Lover (Performed by Miley Cyrus)-pB_V7cwbyY8.mkv"
Deleting original file Happy Hippie Presents - 50 Ways to Leave Your Lover (Performed by Miley Cyrus)-pB_V7cwbyY8.f137.mp4 (pass -k to keep)
Deleting original file Happy Hippie Presents - 50 Ways to Leave Your