The easiest way is to download the video with a firefox add-on like this one:
And then use vlc to extract the audio:
If you are having trouble downloading the video, then you will have to use audacity to record the sound card output using pulse audio:
You can disable the microphone in pauvcontrol by going to the "Input" tab; in the "Show" drop down menu on the bottom, select "All Input Devices"; on the microphone section, click on the little speaker with the red "X", next to it to mute the mike and you can adjust the volume on the "Monitor of Built-in Audio Analog Stereo" section by moving the sliders for "Front Left" and "Front Right". They are set at 100%(0dB) by default.