WARNING: Requested formats are incompatible for merge and will be merged into mkv. The output mkv file has no audio when opened with VLC.

[Ativerc]

0. --verbose output

youtube-dl --verbose https://youtu.be/XrhmepZlCWY
[debug] System config: []
[debug] User config: []
[debug] Command-line args: [u'--verbose', u'https://youtu.be/XrhmepZlCWY']
[debug] Encodings: locale UTF-8, fs UTF-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2016.02.27
[debug] Python version 2.7.6 - Linux-3.13.0-77-generic-x86_64-with-Ubuntu-14.04-trusty
[debug] exe versions: ffmpeg 2.4.3-1ubuntu1, ffprobe 2.4.3-1ubuntu1
[debug] Proxy map: {}
[youtube] XrhmepZlCWY: Downloading webpage
[youtube] XrhmepZlCWY: Downloading video info webpage
[youtube] XrhmepZlCWY: Extracting video information
[youtube] XrhmepZlCWY: Downloading MPD manifest
WARNING: Requested formats are incompatible for merge and will be merged into mkv.
[debug] Invoking downloader on u'https://r4---sn-h557sne7.googlevideo.com/videoplayback?id=5eb8667a96650966&itag=137&source=youtube&requiressl=yes&nh=IgpwcjAyLm1hYTAzKgkxMjcuMC4wLjE&mv=m&pl=20&ms=au&mm=31&mn=sn-h557sne7&ratebypass=yes&mime=video/mp4&gir=yes&clen=96214600&lmt=1458199664016023&dur=408.908&key=dg_yt0&fexp=9407000,9414702,9416126,9420452,9422596,9423607,9423661,9423662,9424092,9425431,9427902,9429131,9429505,9429745,9431012,9431270,9431477,9431859&sver=3&upn=uAFLMC1sEFE&signature=551DE05F2C0B32526C48B45EACB540F19988749A.87ED307CB9DB881BCD2E9CD3B9F5B172F209806B&mt=1458385975&ip=117.245.89.240&ipbits=0&expire=1458407703&sparams=ip,ipbits,expire,id,itag,source,requiressl,nh,mv,pl,ms,mm,mn,ratebypass,mime,gir,clen,lmt,dur'
[download] Destination: Steve Wozniak’s Formative Moment-XrhmepZlCWY.f137.mp4
[download] 100% of 91.76MiB in 25:27
[debug] Invoking downloader on u'https://r4---sn-h557sne7.googlevideo.com/videoplayback?id=o-AMoYx3GFj7Yh9jRlS69WD279YtZX8vALhdRcGLgLrxWq&mm=31&mn=sn-h557sne7&ip=117.245.89.240&expire=1458407699&ms=au&ipbits=0&fexp=9407000%2C9414702%2C9416126%2C9420452%2C9422596%2C9423607%2C9423661%2C9423662%2C9424092%2C9425431%2C9427902%2C9429131%2C9429505%2C9429745%2C9431012%2C9431270%2C9431477%2C9431859&mv=m&pl=20&dur=408.921&keepalive=yes&mt=1458385975&clen=7442205&gir=yes&mime=audio%2Fwebm&key=yt6&nh=IgpwcjAyLm1hYTAzKgkxMjcuMC4wLjE&requiressl=yes&initcwndbps=1617500&lmt=1458057863959996&sparams=clen%2Cdur%2Cgir%2Cid%2Cinitcwndbps%2Cip%2Cipbits%2Citag%2Ckeepalive%2Clmt%2Cmime%2Cmm%2Cmn%2Cms%2Cmv%2Cnh%2Cpl%2Crequiressl%2Csource%2Cupn%2Cexpire&signature=5205297013F966B6D1E5D42C4DBE4A1929B28D71.A546158E391D43F13DF827C4587D1376CCBC0C2C&sver=3&upn=cq6TxaB2PME&source=youtube&itag=251&ratebypass=yes'
[download] Destination: Steve Wozniak’s Formative Moment-XrhmepZlCWY.f251.webm
[download] 100% of 7.10MiB in 02:10
[ffmpeg] Merging formats into "Steve Wozniak’s Formative Moment-XrhmepZlCWY.mkv"
[debug] ffmpeg command line: ffmpeg -y -i 'file:Steve Wozniak’s Formative Moment-XrhmepZlCWY.f137.mp4' -i 'file:Steve Wozniak’s Formative Moment-XrhmepZlCWY.f251.webm' -c copy -map 0:v:0 -map 1:a:0 'file:Steve Wozniak’s Formative Moment-XrhmepZlCWY.temp.mkv'
Deleting original file Steve Wozniak’s Formative Moment-XrhmepZlCWY.f137.mp4 (pass -k to keep)
Deleting original file Steve Wozniak’s Formative Moment-XrhmepZlCWY.f251.webm (pass -k to keep)

1. Problem Description:

The output .mkv file or in this case the Steve Wozniak’s Formative Moment-XrhmepZlCWY.mkv when opened with VLC Media Player gives video but NO sound and gives this message -
No suitable decoder module: VLC does not support the audio or video format "undf". Unfortunately there is no way for you to fix this.

2. What I tried to do:

When the problem first came up a few months ago. I observed that only some videos on YT show this problem. Not all. For e.g. this one [https://www.youtube.com/watch?v=CDsNZJTWw0w] doesn't show this problem.

I read around and found that ffmpeg was a requirement and problem would not show if I had ffmpeg. I checked and found that ffmpeg was already there on my PC. But still the problem showed up.

3. Possible related issues:

These issues are the ones which I read:

1. #8816 [WARNING: Requested formats are incompatible for merge and will be merged into mkv] - The author here doesn't tell about whether the output file has video+audio or not. More concerned with the WARNING itself.
2. #7048 [no sound in youtube video unless the audio .webm gets ffmpeg-ed into .ogg with "-c copy" first] - The author [username ghost] actually has a solution to this and says that "VLC Media Player is the culprit here". But it is pretty confusing

4. What I want:

1. Any of the devs of youtube-dl or anyone who knows about this, please tell me why is there no audio when I try to play the output mkv file? What is causing this problem? youtube-dl, ffmpeg or vlc? Also, please add the answer to the "FAQs/Known Issues" part of youtube-dl's README file. There's very little information about this problem on the Internet.
2. Everything was working fine about a year ago. So what changes did YouTube bring about that is causing this problem?
3. How can I avoid this "No sound" problem? Previously youtube-dl was a "fire-and-forget" type of application. Videos downloaded through it had great audio and video. But now even if there's an output, there's a huge uncertainty. How do I go back to that previous state?

If you need any more details or more description, do tell me.

[dstftw]

1. Update ffmpeg.
2. Update VLC (2.2.2 plays produced file just fine).
3. I you don't want to update use format selection to force selection to mp4+m4a or webm+webm in order to mux not in mkv.
4. Use search. It has been discussed a lot.