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This is useful if you really want to record the audio of a clip, which is not on youtube (you can use youtube-dl there).
The howto explains pretty much everything.
Here are the amixer parameters I needed:
amixer cset numid=24,iface=MIXER,name='Capture Source' 5
~/.asoundrc:
pcm.copy { type plug slave { pcm hw } route_policy copy }
record commandline:
arecord -f cd -t wav -D copy foo.wav