I just came across a scenario where I needed to strip the audio out of a YouTube video file I had created in a Google Hangouts on Air recorded last night for The Unique Geek podcast. I found this post and it did exactly what I needed. Basically, I used the tool avconv via terminal but needed to install the codec necessary to convert to .mp3. The command to install the codec is this:
sudo apt-get install libavcodec-extra-53
I then used the following command to complete the conversion:
avconv -i video.mp4 -ab 192000 audio.mp3