What the real wizard could do is download the free in the park youtube clip and tell you what adjustment he uses to correct it in audacity.
Many thanks for the HP Youtube songs, Ghostwithasmile. They are the best quality I've seen and heard. There is still some slight "quivering"/flutter on Fred's voice but maybe he was nervous or had a sore throat ! I must listen again to the officially-released HP song ("39" I think). With other copies of this "Holy Grail" video, the audio flutter/quiver/waver makes it a hard, unpleasant listening experience, sadly...
Thanks Peter, that's good enough for me.