Is this legal? No idea, sorry.

As to how you might do it, having consulted your lawyers and got full legal clearance ... I'd have thought that recording the audio whilst it plays would be the way to go. Audacity will do this (e.g. record the sound being played on your computer speakers) and the various commercial sound editors (Audition, SoundForge etc.) will do the same. Another alternative is the Freecorder toolbar which sets up as a browser toolbar, although not everyone is happy with having that on their system (I've never found any suspicious activity associated with its presence, others may join this thread and state otherwise).