Avconv warning while downloading YouTube video

To answer OP's question the way he wants it:

# before
$ avconv |& grep \ version | awk '{print $3}'
9.18-6:9.18-0ubuntu0.14.04.1,

$ sudo add-apt-repository ppa:heyarje/libav-11 && sudo apt-get update
$ sudo apt-get install libav-tools

# after
$ avconv |& grep \ version | awk '{print $3}'
11.3-6:11.3-1~trusty,

Alternatively, can avconv -version to get avconv latest version

By default youtube-dl uses avconv, but you can tell it to use ffmpeg instead.

Then use youtube-dl with --prefer-ffmpeg flag, e.g.:

youtube-dl -f 137+140 --prefer-ffmpeg <url>

To avoid typing that flag everytime, you can put it in the configuration file.