can't find matching .m4a for .mp4, downloads .webm audio instead

[pts]

• I've verified and I assure that I'm running youtube-dl 2017.05.18.1
• At least skimmed through README and most notably FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones
• Bug report (encountered problems with youtube-dl)

$ youtube-dl -v -F https://www.youtube.com/watch?v=lw6j4VgzzTc
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'-F', u'https://www.youtube.com/watch?v=lw6j4VgzzTc']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.05.18.1
[debug] Python version 2.7.6 - Linux-4.4.0-75-generic-x86_64-with-Ubuntu-14.04-trusty
[debug] exe versions: avconv 11.3-6, avprobe 11.3-6, rtmpdump 2.4
[debug] Proxy map: {'auto': 'https://proxyconfig.corp.google.com/proxy.pac'}
[youtube] lw6j4VgzzTc: Downloading webpage
[youtube] lw6j4VgzzTc: Downloading embed webpage
[youtube] lw6j4VgzzTc: Refetching age-gated info webpage
[youtube] lw6j4VgzzTc: Extracting video information
[info] Available formats for lw6j4VgzzTc:
format code  extension  resolution note
249          webm       audio only DASH audio   49k , opus @ 50k, 55.85KiB
250          webm       audio only DASH audio   65k , opus @ 70k, 74.16KiB
171          webm       audio only DASH audio   84k , vorbis@128k, 95.82KiB
140          m4a        audio only DASH audio   95k , m4a_dash container, mp4a.40.2@128k, 109.20KiB
251          webm       audio only DASH audio   96k , opus @160k, 109.13KiB
278          webm       82x144     144p   37k , webm container, vp9, 30fps, video only, 42.88KiB
160          mp4        82x144     144p   38k , avc1.4d400b, 30fps, video only, 42.93KiB
242          webm       136x240    144p   77k , vp9, 30fps, video only, 86.77KiB
133          mp4        136x240    144p   81k , avc1.4d400c, 30fps, video only, 91.65KiB
243          webm       204x360    240p  142k , vp9, 30fps, video only, 160.62KiB
134          mp4        204x360    240p  205k , avc1.4d400d, 30fps, video only, 232.26KiB
244          webm       270x480    240p  261k , vp9, 30fps, video only, 295.24KiB
135          mp4        270x480    240p  368k , avc1.4d4015, 30fps, video only, 418.59KiB
247          webm       406x720    360p  528k , vp9, 30fps, video only, 593.70KiB
136          mp4        406x720    360p  740k , avc1.4d401e, 30fps, video only, 838.75KiB
248          webm       608x1080   480p  943k , vp9, 30fps, video only, 1.02MiB
137          mp4        608x1080   480p 1381k , avc1.64001f, 30fps, video only, 1.53MiB
271          webm       720x1280   720p 2253k , vp9, 30fps, video only, 2.46MiB
264          mp4        720x1280   720p 2602k , avc1.64001f, 30fps, video only, 2.88MiB
17           3gp        176x144    small , mp4v.20.3, mp4a.40.2@ 24k
36           3gp        136x240    small , mp4v.20.3, mp4a.40.2
18           mp4        202x360    medium , avc1.42001E, mp4a.40.2@ 96k
43           webm       640x360    medium , vp8.0, vorbis@128k
22           mp4        406x720    hd720 , avc1.64001F, mp4a.40.2@192k (best)

Incorrect default behavior:

$ $ youtube-dl -v https://www.youtube.com/watch?v=lw6j4VgzzTc
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'https://www.youtube.com/watch?v=lw6j4VgzzTc']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.05.18.1
[debug] Python version 2.7.6 - Linux-4.4.0-75-generic-x86_64-with-Ubuntu-14.04-trusty
[debug] exe versions: avconv 11.3-6, avprobe 11.3-6, rtmpdump 2.4
[debug] Proxy map: {'auto': 'https://proxyconfig.corp.google.com/proxy.pac'}
[youtube] lw6j4VgzzTc: Downloading webpage
[youtube] lw6j4VgzzTc: Downloading embed webpage
[youtube] lw6j4VgzzTc: Refetching age-gated info webpage
[youtube] lw6j4VgzzTc: Extracting video information
WARNING: Requested formats are incompatible for merge and will be merged into mkv.
[debug] Invoking downloader on u'https://r3---sn-4g57kn6z.googlevideo.com/videoplayback?expire=1495341374&id=o-AJX2-H8ZmkYKteAwU74mLTr5y9HIagaHaiDGTsNNOWRF&lmt=1495203680131218&ip=77.59.129.137&ei=3sQgWY6fCMLYcJ_9h6AK&pl=20&beids=%5B9466592%5D&initcwndbps=1592500&source=youtube&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cupn%2Cexpire&upn=q8b18zTvYXc&key=yt6&keepalive=yes&clen=3022755&mime=video%2Fmp4&ipbits=0&requiressl=yes&dur=9.299&mv=m&mt=1495319660&ms=au&mn=sn-4g57kn6z&signature=878003A61049171DCA76E40F0E37474BFC0FE7BF.7143889E1B57BADDAE549C69C16F09A176D8C519&mm=31&itag=264&gir=yes&ratebypass=yes'
[download] Destination: Upskirt-lw6j4VgzzTc.f264.mp4
[download] 100% of 2.88MiB in 00:26
[debug] Invoking downloader on u'https://r3---sn-4g57kn6z.googlevideo.com/videoplayback?expire=1495341374&id=o-AJX2-H8ZmkYKteAwU74mLTr5y9HIagaHaiDGTsNNOWRF&lmt=1495079119960962&ip=77.59.129.137&ei=3sQgWY6fCMLYcJ_9h6AK&pl=20&beids=%5B9466592%5D&initcwndbps=1592500&source=youtube&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cupn%2Cexpire&upn=q8b18zTvYXc&key=yt6&keepalive=yes&clen=111753&mime=audio%2Fwebm&ipbits=0&requiressl=yes&dur=9.301&mv=m&mt=1495319660&ms=au&mn=sn-4g57kn6z&signature=1BC3AB446BBFC1C3C47D6B331F0006A814FD9762.74E256E22CAAB709F3292AA854D33230B51AF409&mm=31&itag=251&gir=yes&ratebypass=yes'
[download] Destination: Upskirt-lw6j4VgzzTc.f251.webm
[download] 100% of 109.13KiB in 00:00
[ffmpeg] Merging formats into "Upskirt-lw6j4VgzzTc.mkv"
[debug] ffmpeg command line: avconv -y -i file:Upskirt-lw6j4VgzzTc.f264.mp4 -i file:Upskirt-lw6j4VgzzTc.f251.webm -c copy -map 0:v:0 -map 1:a:0 file:Upskirt-lw6j4VgzzTc.temp.mkv
Deleting original file Upskirt-lw6j4VgzzTc.f264.mp4 (pass -k to keep)
Deleting original file Upskirt-lw6j4VgzzTc.f251.webm (pass -k to keep)

Manual workaround is specifying format numbers:

$ youtube-dl -v -f 264+140 https://www.youtube.com/watch?v=lw6j4VgzzTc
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: [u'-v', u'-f', u'264+140', u'https://www.youtube.com/watch?v=lw6j4VgzzTc']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2017.05.18.1
[debug] Python version 2.7.6 - Linux-4.4.0-75-generic-x86_64-with-Ubuntu-14.04-trusty
[debug] exe versions: avconv 11.3-6, avprobe 11.3-6, rtmpdump 2.4
[debug] Proxy map: {'auto': 'https://proxyconfig.corp.google.com/proxy.pac'}
[youtube] lw6j4VgzzTc: Downloading webpage
[youtube] lw6j4VgzzTc: Downloading embed webpage
[youtube] lw6j4VgzzTc: Refetching age-gated info webpage
[youtube] lw6j4VgzzTc: Extracting video information
[debug] Invoking downloader on u'https://r3---sn-4g57kn6z.googlevideo.com/videoplayback?ms=au&mt=1495319780&mv=m&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cupn%2Cexpire&ip=77.59.129.137&requiressl=yes&id=o-AOGdd2W_AuKQ7XvG791wOUmF2pcuvdNbKlQR3WGMYjSC&pl=20&mm=31&mn=sn-4g57kn6z&expire=1495341457&ipbits=0&dur=9.299&gir=yes&mime=video%2Fmp4&key=yt6&clen=3022755&itag=264&source=youtube&upn=p-agMBAC9aY&lmt=1495203680131218&signature=4FD82CD4B228AA6269EDED34E4FCBA992B82D75E.8723217F7599B493BACCC231211AFBB06055B697&ei=McUgWbrMGomtc5uwt_gL&initcwndbps=1607500&keepalive=yes&ratebypass=yes'
[download] Destination: Upskirt-lw6j4VgzzTc.f264.mp4
[download] 100% of 2.88MiB in 00:02
[debug] Invoking downloader on u'https://r3---sn-4g57kn6z.googlevideo.com/videoplayback?ms=au&mt=1495319780&mv=m&sparams=clen%2Cdur%2Cei%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cpl%2Crequiressl%2Csource%2Cupn%2Cexpire&ip=77.59.129.137&requiressl=yes&id=o-AOGdd2W_AuKQ7XvG791wOUmF2pcuvdNbKlQR3WGMYjSC&pl=20&mm=31&mn=sn-4g57kn6z&expire=1495341457&ipbits=0&dur=9.357&gir=yes&mime=audio%2Fmp4&key=yt6&clen=111822&itag=140&source=youtube&upn=p-agMBAC9aY&lmt=1495203590734521&signature=1C372690369802372788D68126B56639C53BB847.5D1214A66016F824DF5DA1A34BD899BF20921A6E&ei=McUgWbrMGomtc5uwt_gL&initcwndbps=1607500&keepalive=yes&ratebypass=yes'
[download] Destination: Upskirt-lw6j4VgzzTc.f140.m4a
[download] 100% of 109.20KiB in 00:00
[ffmpeg] Merging formats into "Upskirt-lw6j4VgzzTc.mp4"
[debug] ffmpeg command line: avconv -y -i file:Upskirt-lw6j4VgzzTc.f264.mp4 -i file:Upskirt-lw6j4VgzzTc.f140.m4a -c copy -map 0:v:0 -map 1:a:0 file:Upskirt-lw6j4VgzzTc.temp.mp4
Deleting original file Upskirt-lw6j4VgzzTc.f264.mp4 (pass -k to keep)
Deleting original file Upskirt-lw6j4VgzzTc.f140.m4a (pass -k to keep)

Expected behavior: the following command should create an .mp4 file:

$ youtube-dl -v https://www.youtube.com/watch?v=lw6j4VgzzTc

[Hrxn]

The default behaviour is: --format "bestvideo+bestaudio/best".

This means: First priority is getting the best video stream possible, the best audio stream possible, and then merge these into a file.

And in your example format code 251 is bestaudio, as intended. So, working as expected.
Format code 140, what you requested, is ranked lower by youtube-dl, as it should be, from a technical standpoint.

What you can do is something like that:

--format "bestvideo[ext=mp4]+bestaudio[ext=m4a]/bestvideo+bestaudio/best"

Or

--format "bestvideo[ext=webm]+bestaudio[ext=webm]/bestvideo+bestaudio/best"
If you want..

[pts]

Thank you for your suggestions!

This is what would be the best for me:

--format "bestvideo+bestaudio[compatiblewithvideo]/bestvideo+bestaudio/best"

compatiblewidthvideo means: an audio format which doesn't trigger the Requested formats are incompatible warning with the chosen video format.

The reason I prefer this is that better file format compatibility (thus better playability and editability in the future) is more important for me than a minor audio quality improvement.

[remitamine]

duplicate of #10226.