I can't download audio version, if i pass -x -k it works. Otherwise i get the message 'post-process file filename.m4a exists, skipping', in an empty directory too.

[Tapiowski]

youtube > youtube-dl -x -v https://www.youtube.com/watch?v=Ah7ob7y6AtQ
[debug] System config: []
[debug] User config: []
[debug] Command-line args: ['-x', '-v', 'https://www.youtube.com/watch?v=Ah7ob7y6AtQ']
[debug] Encodings: locale 'UTF-8', fs 'utf-8', out 'UTF-8', pref: 'UTF-8'
[debug] youtube-dl version 2014.02.28
[debug] Python version 2.7.6 - Darwin-..-_-__-_****
[debug] Proxy map: {}
[youtube] Setting language
[youtube] Ah7ob7y6AtQ: Downloading webpage
[youtube] Ah7ob7y6AtQ: Downloading video info webpage
[youtube] Ah7ob7y6AtQ: Extracting video information
[download] Destination: Super Momotarou Dentetsu (NES) Music - Unknown Theme O-Ah7ob7y6AtQ.m4a
[download] 100% of 223.95KiB in 00:00
[youtube] Post-process file Super Momotarou Dentetsu (NES) Music - Unknown Theme O-Ah7ob7y6AtQ.m4a exists, skipping

[jaimeMF]

Duplicate of #2288. The audio is downloaded (it's the Super Momotarou Dentetsu (NES) Music - Unknown Theme O-Ah7ob7y6AtQ.m4a file) but it's only playable in mplayer. You can force the conversion to other format using --audio-format aac (or mp3 ...).