Are these from radio broadcasts or just portions of the film soundtracks where there were no concurrent dialogue or effects?

There are apps that can capture the Youtube playback, such as Imtoo Youtube-to-Mp3 converter software, although I don't use it.  I have an external audio interface that I use for midi and sound editing; I use the PC's onboard Soundblaster for non-critical everyday PC use.  I can play back the Youtube via SoundBlaster and capture the audio on the outboard hardware.