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This question already has an answer here:

Can I directly download audio from a YouTube video instead of first downloading audio+video and then extracting the audio?

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marked as duplicate by Maythux, karel, souravc, mikewhatever, Braiam Feb 24 '14 at 2:57

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

I don't think its a duplicate because that question clearly required the search facility. This question does not. I have also mentioned that in the answer. This question is useflu to people who already know what has to be downloaded. – DK Bose Feb 22 '14 at 4:45
The same answer there is applicable her – Maythux Feb 22 '14 at 4:46
Which answer? Can you give the link? – DK Bose Feb 22 '14 at 4:47
@blade answer you can check it – Maythux Feb 22 '14 at 4:48
That answer has to download the video first. This question and answer (1) does not want downloading of the video (2) does not want to search YouTube. I hope you understand. – DK Bose Feb 22 '14 at 5:35
up vote 36 down vote accepted

Here is a recent article in that explains how to use youtube-dl to directly download audio instead of first downloading video+audio and then extracting audio using -x. Unfortunately, the search facility that Maythux asked for here isn't offered. But it is still worth a look:
Video Downloader youtube-dl Gets Support For Separate Audio And Video YouTube DASH Streams

Basically, download the latest version of youtube-dl from source or from the ppa offered in the link above but not from the Software Center.

Then, if you already have the video link ...

Run, as example:

youtube-dl -F

This will list the various download formats available for this url (audio and video).

$ youtube-dl -F
[youtube] Setting language
[youtube] HRIF4_WzU1w: Downloading webpage
[youtube] HRIF4_WzU1w: Downloading video info webpage
[youtube] HRIF4_WzU1w: Extracting video information
[info] Available formats for HRIF4_WzU1w:
format code extension resolution  note 
171         webm      audio only  DASH webm audio , audio@ 48k (worst)
140         m4a       audio only  DASH audio , audio@128k
160         mp4       192p        DASH video 
133         mp4       240p        DASH video 
134         mp4       360p        DASH video 
135         mp4       480p        DASH video 
17          3gp       176x144     
36          3gp       320x240     
5           flv       400x240     
43          webm      640x360     
18          mp4       640x360     (best)

Now, choose desired audio format. I went for 140


youtube-dl -f 140

$ youtube-dl -f 140
[youtube] Setting language
[youtube] HRIF4_WzU1w: Downloading webpage
[youtube] HRIF4_WzU1w: Downloading video info webpage
[youtube] HRIF4_WzU1w: Extracting video information
[download] Destination: Martin Luther King - I Have a Dream on August 28, 1963 [Sous-titres & Subtitles] [FULL SPEECH]-HRIF4_WzU1w.m4a
[download] 100% of 15.19MiB in 00:04

That's it.

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Some audio players do not support DASH audio. For m4a, I had to demux it to make it a 'regular' m4a using ffmpeg. For example: ffmpeg -i input.m4a -vn -c:a copy output.m4a – Sun Nov 7 '14 at 8:14
@sunk818 and just a public announcement that: youtube-dl 2015.01.23.4 and newer will automatically correct the header of the m4a file if ffmpeg is present on the system. So, if ffmpeg is installed before youtube-dl is called now it will automatically fix the DASH container and you won't need to run sunk818's fix. – tsujp Apr 21 '15 at 16:15
Yes, I am familiar with that fix. Another youtube-dl user and I posted an issue, one of the youtube-dl coders read it, and put in an enhancement for us. – Sun Apr 21 '15 at 16:53
youtube-dl -f bestaudio http://... if you don't want to select best audio manually. – Seppo Erviälä Nov 19 '15 at 13:27
be careful with -f bestaudio as best audio could be .webm and not .m4a Use -f m4a if you want best m4a quality audio. – Sun Apr 13 at 23:31

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