There is no need to install any extra software.
First, set your PC to record from the wave out mix...
1.) Open volume control (double click speaker icon in the taskbar)
2.) Goto options/properties
3.) press the recording button then ok
4.) click the select box below Wave Out Mix and turn it's level up.
Next open Sound Recorder. It's in the Start Menu, Accessories, Entertainment. You need to do a save first to set the correct format. Press record then play the video.
You can use this method to record any audio playing on your sound card. ie. Youtube, Skype, games etc. etc. You can also use this method to convert DRM protected audio files into wave files. Technically that means Microsoft Windows violates the DCMA and should be illegal since it assists in bypassing a copy protection scheme.