Well I found these 2 web pages, if it helps anyone in the future:
Recording Your Computer’s Audio
An alternative to the use of a secondary piece of kit is to take advantage of the marvelous Audacity, available from here
After downloading and installing this software, you can use it as a digital studio for compiling tracks or as a useful tool for recording audio from source on your PC.
This is done using the options on the Audacity Device Toolbar, which you will find just below the play
options on the Audacity Transport Toolbar. Here, set the source option – indicated by a microphone symbol – to Stereo Mix
(the option will have your sound card listed as a suffix).
With this selected, click the record
button and press play on the video clip you wish to record. When you’re done and you listen back, you might notice that the sound is a little quieter than it was when you played it as a video clip. This is likely due to the audio settings on the video player; use Effect > Amplify
in Audacity to boost this up a little.